Icon Sets allow you to package multiple toolbar icons into a single file for easy sharing and faster startup. They also allow you to replace internal icons (without editing individual toolbar buttons), and to give your icons names and categories which the user can search.
The specifications for the Directory Opus Icon Set Format are documented in the manual:
https://www.gpsoft.com.au/help/opus12/index.html#!Documents/Icon%20Sets/Icon_Sets.htm