MX-FXX1 (serv.man5). Fax Guide (Supplied as pdf file on m c's Hard Disk Drive). - Sharp Copying Equipment User Guide / Operation Manual. Page 109

By sending a fax to an F-code memory box (confidential) in the receiving machine (your machine or the other machine), 
the sender can direct the transmission specifically to the user of the box.
This is convenient for sending sensitive documents that you do not wish people other than the recipient to see, or when 
the receiving machine is shared by multiple departments.
To print an F-code confidential fax, the print passcode must be entered.

The F-code (sub-address and passcode) of the memory box to be used should be verified by the sender and the recipient 
before the fax is sent.

System Settings for Fax: F-code Memory Box
This is used to create memory boxes for F-code confidential communication (confidential).
A memory box name, sub-address, passcode, and print passcode are programmed in each box.

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Sub-address: AAAAAAAA
Passcode: XXXXXXXX

Sub-address: AAAAAAAA
Passcode: XXXXXXXX

Reception in F-code Memory Box

The fax is printed.