choices in general can take arbitrary Lisp objects as the items , and then the behavior of the items (how they are displayed, callbacks) is determined by the properties of the choice. It is possible to give individual properties to individual items by using objects of class item, which encapsulates the properties of an item in a choice. The items of a choice can be a mixture of arbitrary objects and item instances.
item has several subclasses which are intended for specific choice subclasses, and these are documented in the entries for the specific choices. The predicate itemp determines whether its argument is an instance of item.
CAPI User Guide and Reference Manual (Macintosh version) - 3 Aug 2017