Know applcust.txt

The $APPL_TOP/admin/applcust.txt file should be used to register your customization and being used as a reference while applying patches.

There are two ways of doing a customization

Customization by Extension (CBE)
Customization by Modification (CBM)

Customization by Extension (CBE) is nothing but doing customizations to copies of the original Oracle applications files.
Customization by Modification (CBM) is doing the customizations directly in the Oracle applciations files. It is difficult to maintain

There is a format to be followed while registering your customization

#      

It is easier to understand the format with an example.Consider that the file $AR_TOP/forms/US/ARXSUVAT.fmb has been modified in the method CBE as $XXAR_TOP/forms/US/XXARXSUVAT.fmb

is nothing but the Standard product abbreviation. In simple meaning the Sprd is taken using the format $_TOP.
is always whatever after $_TOP, in our case it is forms/US
is the exact file name.
is your custom top where you have your customized/modified file.
is the directory structure after $_TOP
is the name of your custom file.

So in our example case, the format of the entry will be as follows
# ar  forms/US    ARXSUVAT.fmb   xxar    forms/US    XXARXSUVAT.fmb

If you have customized your file in the CBM method, then you can leave the last three entries as blank (as below)

# ar  forms/US    ARXSUVAT.fmb

The applcust.txt is replaced by Register Flagged Files Tool in R12. Register Flagged Files tool can be accessed through Oracle Applications Manager (OAM) responsibility in R12.