If we wish to apply the rtu def, then we apply rule 1a. However, when we walk the supertype tree, we see that it subtypes from two mandatory defs: ahu and equip. Therefore, to implement rtu requires adding all three tags: rtu, ahu, equip. As a general rule, top-level entity markers such as site, space, equip, point are always required. This makes it easy to query your tag based data.