Tags can be thought of as property values for an instance of an object. Adding a tag is like extending an object definition by adding a new property and setting a value for an instance of that object. The difficulty is in defining the relevant properties of the entire object up-front. The intent and purpose and audience must be understood from the usage start or else there must be the capability to evolve the definition (this may break some of its historical usages though). In some cases, it could be helpful to associate a "measurable degree" to the property which indicates just how much the value assigned to the object is a good fit relative to a defined list of possible values. The category and subcategories should always be in the set of tags for an object in some way, shape or form.