I cannot sign, I get the message “You selected a .cer file that corresponds to a CSD”

What is a CSD and how is it different from an e.firma?

The e.firma and the CSD may sound like the same thing, since both are composed of a private key (.key file) protected by a password and a public certificate (.cer file).

However, there is a legal and technical difference between the two. The e.firma can be used to “sign” any data message or digital document, but the CSD can only be used to issue or “stamp” digital invoices.

How can I identify a CSD?

In general, the e.firma files can be easily differentiated from the CSD files, since the former are usually named as 'FIEL', 'efirma' or your RFC, while the latter are named 'CSD', 'SELLODIGITAL' or the name of the branch that will issue the invoices.

Another way is to verify it in the SAT portal. Just enter your RFC and verify which of your certificates correspond to an e.firma (shown as “FIEL”) and which to a CSD (shown as “Sello”).

My accountant takes care of all that, what files should I ask them for in order to use Mifiel?

Tell your accountant that you need your FIEL or e.firma files, not the sello digital CSD files.