Fax survives where a signed document has to reach a named party and leave a record on both ends. Often a rule written before email was trusted says so. This post gives the reasons with the rule behind each. Pharmacies and clinics first, then courts, then lenders and the tax office. The last section is the technical property those rules lean on.
The rules name fax
The United States Drug Enforcement Administration's regulation on Schedule II prescriptions, 21 CFR 1306.11, names facsimile in three exceptions. Paragraph (e) allows a practitioner, or the practitioner's agent, to fax a Schedule II prescription to the pharmacy. It covers a narcotic substance compounded for infusion, including home infusion. Paragraph (f) allows the same for any Schedule II substance for a resident of a long term care facility. Paragraph (g) allows it for a narcotic substance for a patient enrolled in a hospice program. The prescription must say the patient is in hospice care.
Each of those three paragraphs says the facsimile is the original written prescription for its own case.
Outside those exceptions, paragraph (a) still lets a prescriber fax a paper prescription ahead. The pharmacist must then see the original manually signed prescription before dispensing.
For a pharmacy that takes those prescriptions by fax, the consequence is practical. It has to receive faxes, so the prescriber has to be able to send one. That gives a clinic working with the pharmacy a reason to keep fax available.
Health privacy law applies to all of it. Fax for Agents is not a HIPAA service and its acceptable use policy rules out protected health information.
Courts
California is a worked example of a court system with fax written into procedure. Rule 2.300 of the California Rules of Court applies the fax filing rules to civil, probate and family law proceedings in all trial courts. Wills, codicils, bonds and undertakings cannot be filed by fax.
Direct filing depends on the individual court. Rule 2.304 allows a party to fax documents directly only to a court whose local rule provides for it. Read the local rule before choosing fax for a deadline.
The same rule says what the filing looks like. Form MC-005, the fax cover sheet, goes first. Each document carries the phrase "By fax" immediately below its title. The filer must have the fax machine print a transmission record of each filing.
That printout has a purpose when something goes wrong. A faxed document can go unfiled because of a transmission error unknown to the sender. It can also go unfiled because the court failed to process it after receiving it. The sender can then move for an order filing it nunc pro tunc, with the earlier date. The rule gives the opportunity to ask. It does not promise the court will grant it.
Service has its own conditions. Rule 2.306 permits service by fax only if the parties agree and confirm the agreement in writing. The proof of service must state that the transmission was reported as complete and without error, and must attach a copy of the transmission report. Service is complete when the entire document reaches the receiving machine. After 5 p.m. it counts as the next court day.
Those requirements explain why a transmission report belongs in the case file. The machine produces the evidence the procedure asks you to keep.
Banks, lenders and the tax office
The IRS instructions for Form SS-4 let you apply for an employer identification number by fax through the Fax-TIN program. The instructions say you can receive your EIN by fax within about four business days. Fax-TIN is available 24 hours a day, 7 days a week. They ask you to include a return fax number so the agency can fax the EIN back. The fax number carries the reply as well as the application.
For lenders, the IRS Income Verification Express Service lets a taxpayer authorize banks and lenders to obtain tax records for a mortgage or loan application. Participants submit transcript requests online or by fax with Form 4506-C.
A lender that chooses the fax route builds its process around a signed consent form and a transmission record. The consent says whose records the lender is authorized to request. The fax carries that signed form to the agency's number. Keeping the form with the send record gives the lender something specific to check when a request stalls. The reason to keep the route is compatibility with the office receiving the request.
What the rules are leaning on
A Group 3 fax is a telephone call between two machines, and ITU-T T.30 defines the session. The sender dials. The receiver answers with tones and the machines negotiate speed and resolution. Pages go across, and after each one the receiver answers with a message confirmation, MCF. The sender's transmission report records that confirmation. T.30 also defines an error correction mode that retransmits damaged blocks.
The confirmation is the property the rules lean on. A message in your sent folder shows that your mail client handed it on. A transmission report shows that the receiving machine took the pages. Neither proves that a person read them.
The page itself is a fixed monochrome image, defined by ITU-T T.4. Standard resolution is 204 by 98 dots per inch. Fine is 204 by 196. What was sent is what prints at the far end, with the signature where it was on the page. Small text and faint handwriting are the things to check before sending.
The number dialled is tied to a line or a provider account. A display name in an email header can be set to anything. A fax call goes where the number goes.
None of that makes fax secure. The basic T.30 exchange is not encrypted, and the receiving machine does not check who is calling. A wrong number sends your page to a stranger, and a successful transmission to that number does not make it the right recipient.
What this means if you are building software
If your software handles one of these workflows, the fax leg is a step you have to plan for, with evidence you can keep. A fax API gives you that step as three records. A quote before the send, with the price for that document to that number. A job with states, from queued through active to the outcome. A receipt with the provider's reference and the hash of the exact document that went out. The mechanics are in how a fax API works.
Keep the document and the receipt together, so the outcome stays tied to the file you sent. Track what the receiving office does next separately in your own system.
An AI agent that ends a workflow at a fax number needs the same receipt. It also needs permission to send that exact document and nothing else.