I have an item that, among its other fields, contains a variable number (0-20, if it's of any consequence) of items that themselves have 3 fields (Year, Number, Number). These "inner" items (akin to previous years' performances) do not make sense to edit on their own, outside of the context of the outer item's details.
In attempting to allow these fields to have these multiple values, I've found that only lookup and person/group items may have 'multiple values'. I have been able to make a list for the inner items to reside in, and am able to select these items using a lookup field I've created. However, creating the items in a separate form is inadequate for my use case, and furthermore, having to select them from a list of all the other items' previous years' performances is a giant mess.