Item Categories are used to organize the Item Definitions. The editor allows you to create and edit your existing Item Categories. The Item Category editor . . .
opsive.com
Make sure you understand the difference between Immutable/mutable & common/unique as it really changes how items are handled throughout the system.
Note that if you want you can create a custom Item Collection script which can stack items however you wish, it's a bit more advanced but it gives you absolute control which can be nice if you make anything a bit out of the ordinary.