You may need an RFC to register your business, a certificate to issue invoices and a different credential to sign a tax procedure. Before starting a procedure, check which credential it accepts and keep the acknowledgment when you finish.
Use each answer as a verification path
For any SAT question, record the RFC, tax period, obligation and source consulted before acting. If one of those elements changes, the answer may also change. This small habit keeps a general explanation from being applied to the wrong taxpayer or period.
What is the RFC?
The RFC is the Mexican federal taxpayer registration key assigned to an individual or legal entity. The registration record also contains activities, tax regimes, obligations, status and tax address.
The RFC alone does not show that every obligation is correct. Review the current tax-status certificate after incorporation, a change of address, a new activity or a regime update.
When should RFC activities or obligations be updated?
File an update when the facts recorded by SAT no longer match the operation, including a new activity, a discontinued activity, a change in how income is earned or a move that changes the tax address.
Before submitting an update, confirm what changed and when it happened. Check whether the change also affects returns you need to file. Keep the submitted notice and its acknowledgment so you can later explain the dates and obligations recorded with SAT.
Is an individual the same taxpayer as a company?
No. An individual files under their own RFC. A Mexican legal entity has a separate RFC, legal name, capital, representatives and obligations.
When deciding whether to incorporate, review who will own the business, who will sign contracts, whether it will hire employees and which tax rules apply. Consider those questions alongside expected revenue.
What is the difference between Password, e.firma and CSD?
| Credential | Main use | Control |
|---|---|---|
| SAT Password | Access to services that expressly accept it. | Keep it personal and change it after suspected exposure. |
| e.firma | Electronic signature for SAT filings, applications and other procedures. | Protect the private key, certificate and private-key password. |
| Digital Seal Certificate, or CSD | Seal CFDI issued by the taxpayer. | Limit it to authorized invoicing systems and staff. |
CFDI are sealed with the CSD, not the e.firma. The e.firma may be needed to obtain or renew the CSD and to complete other SAT procedures.
Never send private keys or passwords through email or chat. A service provider should receive only the access needed for its assigned task.
What data is needed to issue CFDI 4.0?
The receiver should provide:
- RFC;
- registered name or legal name;
- receiver tax regime;
- fiscal postal code;
- intended CFDI use.
The issuer must also select the correct product or service key, unit, description, amount, taxes, export code, payment method and payment form. Customer data should match SAT records exactly.
The XML is the tax document. The PDF is a readable representation and cannot replace the XML in the accounting file.
What is the difference between PUE and PPD?
| Method | Meaning | Follow-up |
|---|---|---|
| PUE | Full payment received by issuance, or valid use of rule 2.7.1.39 for same-month payment. | Record the actual payment form and reconcile the collection. |
| PPD | Payment was not received in full at issuance. | Use payment form 99 and issue a payment complement as collections occur. |
Payment complements are due by the fifth calendar day of the month after collection. They must identify the invoice UUID, payment date, amount and balances.
What should be done when a CFDI is wrong?
Identify whether the transaction continues and whether the new invoice must be linked to the old one.
| Cancellation reason | Use |
|---|---|
| 01 | Error with a linked replacement. |
| 02 | Error without a replacement relationship. |
| 03 | Transaction did not occur. |
| 04 | Named transaction included in a global invoice. |
For reason 01, first issue the corrected CFDI and relate it to the original using relationship type 04. Then request cancellation of the original with reason 01 and identify the replacement UUID. Check the final cancellation status and retain the acknowledgment.
Do monthly returns always fall on the same date?
No. Many provisional or final federal tax payments use the 17th day of the following month, and a federal decree may add business days according to the sixth numeric digit of the RFC. Other obligations follow different calendars, periodicities or special regimes.
Build the filing calendar from the active obligations in the RFC and the current service page. Do not copy the due date from another taxpayer.
What evidence should be kept for a monthly filing?
| Evidence | Purpose |
|---|---|
| Tax workpaper | Explains the tax base, credits and withholding. |
| CFDI inventory | Supports income, expenses, cancellations and collections. |
| Bank reconciliation | Connects cash movement to the accounting records. |
| Return acknowledgment | Confirms SAT received the return. |
| Payment reference | States amount and payment validity period. |
| Bank receipt | Proves the tax payment was received by the bank. |
An acknowledgment confirms receipt of the return. When tax is due, the bank receipt is separate payment evidence.
What is the SAT Tax Mailbox?
The Tax Mailbox is the electronic system through which SAT can notify administrative acts and taxpayers can submit certain requests, notices or responses.
Registered email addresses and a mobile number receive alerts. The alert is not the administrative act. Enter the mailbox, open the pending item, download the act and keep the notification acknowledgment.
Article 17-K of the Federal Tax Code requires taxpayers to consult the mailbox within three days after an electronic alert reaches a registered contact method.
What is a tax-compliance opinion?
The opinion reports the status that SAT systems show under rule 2.1.36 of the 2026 Miscellaneous Tax Resolution. Its formal results are positive, negative, suspended activities or registered without tax obligations.
A positive opinion lasts 30 calendar days from issue. It does not constitute a ruling that the tax calculations or amounts paid are correct.
What should an SMB reconcile each month?
- issued CFDI against sales and collections;
- received CFDI against purchases and payments;
- cancellations and replacements;
- payment complements against bank deposits;
- payroll CFDI against payroll entries and withholding;
- VAT and ISR workpapers against the ledger;
- returns, payment references and bank receipts;
- Tax Mailbox items and response deadlines.
For each unexplained difference, record the amount, the missing information and who will resolve it. For example, an unidentified deposit may need confirmation from the customer before you can match it to an invoice. Review these pending items before completing the month's calculations.
When is professional review needed?
Seek tax or legal review when:
- the RFC shows an activity or obligation that does not match the business;
- a filed return differs from CFDI or banks;
- several CFDI must be canceled or replaced after filing;
- an unrecognized tax liability or negative compliance opinion appears;
- the Tax Mailbox contains a request, assessment or deadline;
- the business adds partners, employees, foreign transactions or another income stream;
- a regime change or amended return is being considered.
Bring the notice, return or invoice that raised the question, together with related bank records. Ask the reviewer to explain which periods are affected and what they propose to correct before submitting changes.
Official sources
- SAT: tax-status certificate.
- SAT: Anexo 20 and CFDI 4.0 format.
- SAT: CFDI cancellation process.
- SAT: Tax Mailbox.
- SAT: obtain a tax-compliance opinion.
- SAT: 2026 Miscellaneous Tax Resolution.
- Chamber of Deputies: Federal Tax Code.
Official links to complete the task
Open official services from a new browser tab, confirm the taxpayer and period, and save the resulting acknowledgment or evidence with the case.
