FN_AUDIT is a SAP Parameter attribute which is used to control Name of security audit file information. This is available within R/3 SAP systems depending on the version and release level.
Below is the standard documentation available and a few details of the attributes values .
Valid file name for the relevant operating system. The name must contain
exactly eight (8) '+' in a row as these contain the current date and the
following six (6) '#' from the system which are then subsituted by a
sequential number.
See SAP Parameter Documentation for full SAP documentation for this profile parameter.