The LATEX 2.09 set-up allowed the format itself to be customised. When making the format with iniTEX, the process ended with this request.
Input any local modifications here.If your site did input any modifications at that point, the LATEX2e `compatibility mode' will not fully emulate LATEX 2.09 as installed at your site.
In this case you should put all these `local modifications' into a file called |latex209.cfg| and put this file in the default input path at your site. These `local modifications', although not stored in the format, will then be loaded before any old-style document is processed. This should ensure that you can continue to process any old documents that made use of this local customisation.