# Fax and fax API glossary

> Short definitions of fax protocol and fax API terms, each with its source and the date it was read. T.30, T.38, Group 3, ECM, CSID and job states.

Page: https://faxforagents.com/glossary/
Last changed: 2026-11-11

Updated Nov 11, 2026

Nineteen terms from fax and from the Fax for Agents API. Each one has two or three sentences you can quote. Under it is the source we checked it against and the day we read it.

An entry states only what the source named under it says. Protocol terms come from ITU-T pages. API terms come from our own documentation and the OpenAPI file.

**Cite this page:** Fax for Agents, "Fax and fax API glossary", [https://faxforagents.com/glossary/](https://faxforagents.com/glossary/), last updated 2026-11-11. To cite one term, add its anchor to that address, as in `#partial`.

## Fax protocol terms

### Group 3 fax

Group 3 is the class of fax terminal that ITU-T T.4 standardizes. Group 3 terminals send black and white documents, and optionally colour ones, over the general switched telephone network. ITU-T T.30 defines the procedures they use.

Source: [ITU-T T.4 (07/2003)](https://www.itu.int/rec/T-REC-T.4-200307-I/en), "Standardization of Group 3 facsimile terminals for document transmission". Read the summary on 2026-11-11. Anchor: `#group-3`.

### ITU-T T.4

ITU-T T.4 is the recommendation that defines the characteristics of Group 3 fax terminals. Its title is "Standardization of Group 3 facsimile terminals for document transmission". The edition in force is 07/2003, approved on 2003-07-14.

Source: [ITU-T T.4 (07/2003)](https://www.itu.int/rec/T-REC-T.4-200307-I/en), "Standardization of Group 3 facsimile terminals for document transmission". Read the edition page and the summary on 2026-11-11. Anchor: `#t-4`.

### ITU-T T.30

ITU-T T.30 defines the procedures that Group 3 fax terminals use to send a document. The procedures cover the general switched telephone network, international leased circuits and ISDN. The edition in force is 09/2005, approved on 2005-09-13.

Source: [ITU-T T.30 (09/2005)](https://www.itu.int/rec/T-REC-T.30-200509-I/en), "Procedures for document facsimile transmission in the general switched telephone network". Read the edition page and the summary on 2026-11-11. Anchor: `#t-30`.

### ITU-T T.38

ITU-T T.38 defines the procedures for Group 3 fax when part of the path between the terminals is an IP network. Its title is "Procedures for real-time Group 3 facsimile communication over IP networks". The edition in force is 11/2015, approved on 2015-11-29.

Source: [ITU-T T.38 (11/2015)](https://www.itu.int/rec/T-REC-T.38-201511-I/en), "Procedures for real-time Group 3 facsimile communication over IP networks". Read the edition page and the summary on 2026-11-11. Anchor: `#t-38`.

### Error correction mode (ECM)

Error correction mode, ECM, is an optional mode of Group 3 fax. ITU-T T.4 lists it as Annex A, "Optional error correction mode". ITU-T T.30 gives the procedure in its Annex A, for Group 3 transmission "incorporating error correction".

Sources: [ITU-T T.4 (07/2003)](https://www.itu.int/rec/T-REC-T.4-200307-I/en), "Standardization of Group 3 facsimile terminals for document transmission". [ITU-T T.30 (09/2005)](https://www.itu.int/rec/T-REC-T.30-200509-I/en), "Procedures for document facsimile transmission in the general switched telephone network". Read the contents of both on 2026-11-11. Anchor: `#error-correction-mode`.

### Standard and fine resolution

Standard and fine are two fax resolutions. Standard is 204 by 98 dpi and fine is 204 by 196 dpi. In the Fax for Agents API, a quote's `quality` field takes `standard`, `fine`, `superfine` or `ultra`, and fine is the default.

Source: [Send a fax](https://faxforagents.com/docs/send-fax/#quotes). The summary of ITU-T T.4 (07/2003) names "support of all standardized image resolutions" and lists no values. Read on 2026-11-11. Anchor: `#standard-and-fine-resolution`.

### CSID

CSID stands for called subscriber identification. ITU-T T.30 abbreviates it CSI and defines it as an optional signal from the called terminal to the calling one. The signal carries the called subscriber's international telephone number in a field of 20 digits.

Source: [ITU-T T.30 (09/2005)](https://www.itu.int/rec/T-REC-T.30-200509-I/en), "Procedures for document facsimile transmission in the general switched telephone network". Read clauses 5.3.6.1.1 and 5.3.6.2.4 on 2026-11-11. Anchor: `#csid`.

## Fax for Agents API terms

The API has six job states: `queued`, `active`, `partial`, `failed`, `unknown` and `delivered`. The names come from the `FaxJob.state` enum in [openapi.json](https://faxforagents.com/openapi.json).

### Job state

A job state tells your application what happened to a fax job. A send answers 201 with a job in `queued`. What happens after is a state on the job, carried by events and webhooks, not an HTTP error.

Sources: [Errors and job states](https://faxforagents.com/docs/errors/#why-is-a-failed-fax-not-an-http-error). [What a fax API is and how one works, from PDF to T.30](https://faxforagents.com/blog/what-is-a-fax-api/). [openapi.json](https://faxforagents.com/openapi.json). Read on 2026-11-11. Anchor: `#job-state`.

### queued

A job in `queued` was accepted and is waiting for the worker. The quoted amount is held in the wallet.

Source: [Send a fax](https://faxforagents.com/docs/send-fax/#what-are-the-six-job-states). Read on 2026-11-11. Anchor: `#queued`.

### active

A job in `active` is with the provider, and attempts are in progress. Busy and no answer are retried up to three times. The amount stays held.

Source: [Send a fax](https://faxforagents.com/docs/send-fax/#what-are-the-six-job-states). Read on 2026-11-11. Anchor: `#active`.

### partial

A job in `partial` had some pages confirmed and not the rest. The confirmed pages are captured and the rest of the hold is released. The service never retransmits a partial job on its own.

Sources: [Send a fax](https://faxforagents.com/docs/send-fax/#what-are-the-six-job-states). [Errors and job states](https://faxforagents.com/docs/errors/#why-is-a-failed-fax-not-an-http-error). Read on 2026-11-11. Anchor: `#partial`.

### failed

A job in `failed` ended unsuccessfully, with no send charge. The cause is exhausted attempts, an invalid number, a render failure or a cancel, and `failure_code` says which. The hold is released.

Sources: [Send a fax](https://faxforagents.com/docs/send-fax/#what-are-the-six-job-states). [Errors and job states](https://faxforagents.com/docs/errors/#why-is-a-failed-fax-not-an-http-error). [What a fax API is and how one works, from PDF to T.30](https://faxforagents.com/blog/what-is-a-fax-api/). Read on 2026-11-11. Anchor: `#failed`.

### unknown

A job in `unknown` has no outcome by the deadline. The hold stays while the reconcile tick keeps asking the provider. The service never retransmits an unknown job on its own.

Sources: [Send a fax](https://faxforagents.com/docs/send-fax/#what-are-the-six-job-states). [Errors and job states](https://faxforagents.com/docs/errors/#why-is-a-failed-fax-not-an-http-error). Read on 2026-11-11. Anchor: `#unknown`.

### delivered

A job in `delivered` had every page confirmed, with the evidence stored on the attempt. The held amount is captured. The state says nothing about whether a person read the document.

Sources: [Send a fax](https://faxforagents.com/docs/send-fax/#what-are-the-six-job-states). [What a fax API is and how one works, from PDF to T.30](https://faxforagents.com/blog/what-is-a-fax-api/). Read on 2026-11-11. Anchor: `#delivered`.

### Idempotency key

An idempotency key is a value you send in the `Idempotency-Key` header of a mutating call. Retry after a timeout with the same key and you get the same job. A different body under the same key is a 409.

Sources: [Send a fax](https://faxforagents.com/docs/send-fax/#the-send). [Errors and job states](https://faxforagents.com/docs/errors/#how-do-i-avoid-sending-a-fax-twice). Read on 2026-11-11. Anchor: `#idempotency-key`.

### Quote

A quote gives you a price before you send. `POST /v1/fax-quotes` prices one document for one destination. The quote fixes the destination, sender number, pages, cover sheet and amount for ten minutes.

Sources: [What a fax API is and how one works, from PDF to T.30](https://faxforagents.com/blog/what-is-a-fax-api/). [Send a fax](https://faxforagents.com/docs/send-fax/#quotes). Read on 2026-11-11. Anchor: `#quote`.

### Approval

An approval is a person's yes to one exact send. It covers one document hash, one destination, one sender, one amount and an expiry. Any change invalidates it.

Source: [Approvals](https://faxforagents.com/docs/approvals/). Read on 2026-11-11. Anchor: `#approval`.

### Delegated policy

A delegated policy is a set of limits an administrator writes once. Every send under it is evaluated on the server from the quote and the document row. Nothing an agent writes is an input, so a prompt cannot move a limit.

Source: [Delegated policies](https://faxforagents.com/docs/policies/). Read on 2026-11-11. Anchor: `#policy`.

### Sandbox

The sandbox takes every send made with a test key. There is no carrier and no cost, and nothing leaves the box. The outcome is decided by the destination number or by a scenario you select.

Source: [Sandbox](https://faxforagents.com/docs/sandbox/). Read on 2026-11-11. Anchor: `#sandbox`.

## How we check a term

For a protocol term we open the recommendation's page on itu.int and write down what it says. Most entries come from the summary of the edition in force. Error correction mode comes from the contents pages, because the summaries do not name it. CSID comes from two numbered clauses of T.30. The resolution entry cites our own send page, because the T.4 summary lists no values.

For an API term we copy the wording of the page named under it. The job state names are read from [openapi.json](https://faxforagents.com/openapi.json), and our build fails when this page and that file differ.

If an entry no longer matches its source, tell us through the [contact page](https://faxforagents.com/contact/) and name the anchor.