That's not the name of exit, it's the name of the exception. If you check the subroutine where the exception is thrown, the descrition is:
*----------------------------------------------------------------------*
* FORM MERGE_ALLKSSK_ALLAUSP
*----------------------------------------------------------------------*
* Merge allocation and valuation tables from user exit (L_*)
* with corresponding tables in standard program (G_*).
*----------------------------------------------------------------------*
If this was my system I would put a break-point inside the perform, and debug.
* take over user changes into allkssk, allausp
perform merge_allkssk_allausp
tables allkssk
lt_allkssk
allausp
lt_allausp.
It tried looking at the code for the name of the exit, but it's a dynamic function call, so you can only see the name in runtime.
EDIT: The actual code where the exception is thrown is:
if l_allausp-atcod na '123456789'.
raise cust_exit_ausp3.
endif.