If you need to modify already existing framework Perl module files or tt template files
or unit test files for customizations, you can do this in the $OTRS_HOME/Custom/ directory.

Example:

  You need to modify Kernel/System/Service.pm to add an customization, copy
  Kernel/System/Service.pm to Custom/Kernel/System/Service.pm and modify it.
  This module will be used instead of the original Service.pm.

This method also works for tt files. Just place them in Kernel/Output/HTML/$ThemeName,
and they will be used if the theme is selected.

It also works for unit test files. Just place them in Custom/scripts/test.

If you use Apache + mod_perl or PerlEx, any Perl module files placed in this directory
will only be loaded by your web server after you restarted it.
