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In addition to ICOand ICNS, there is a third called iContainer, developed by Panic Software, created to package sets of Mac icons. It supports icons up to 512px and includes display settings and information about each icon. It is primarily used in Panic’s CandyBar app for organising icons and customising Mac OS X system and application icons.
The iContainer format contains a set of icons, including information like the title and designer, and even the colour of background they should be displayed on in apps like CandyBar