The UI interface for a content item has a "tab" for managing tag associations.

That tab displays all categories that can apply to items of this content type.

Categories are containers of sub-categories or tags. Tags have an IOBTree attr where each item is an association with a content item keyed by the integer epoch date of the association. So when you query a tag you get the items ordered by date, which is a very common ordering.

Tag cardinality: single tags, multi tags.

class ICategory(Interface):
    def isApplicableTo(contentType):
        '''Is this tag applicable to this content object'''
    multivalued = Attribute('''True if multiple tags of this category can be applied to an item''')
    hierarchical = Attribute('''If true if this category can contain sub-categories; otherwise, it can contain only tags.''')
        
class ITag(Interface):
    def isApplicableTo(content):
        '''Is this tag applicable to this content object'''
    associations = Attribute('''IOBtree where each key is the epoch date of when the association was created and the value is the unique id of the content item''')