Are you asking about the IOS image, or the your configuration files? If you're asking about the IOS image, then that's assigned by Cisco and should be documented. If you're asking about your config files, then that's a standard you set and follow on your own.

For my PIX configurations, I use:

pix-YYYYMMDD-FEAT1:FEAT2:...:FEATN.conf

...it gets lengthy, but it works!

-C