Some notes:

• The full details of an entity may not be contained in the Registers. It is important (especially when creating / writing instances of Registers entries) to consult the full details of entries in the relevant Defining Documents (for example, the values permitted for an instance of an Element may be very restricted yet there may be no restrictions recorded in the Elements Register).
• Some entries are classed as NODEs which just means that they are not complete entries – they are present just as “dummy” entries that act as “markers” in the UL space of the Register.
• It is possible to form a hierarchy of entries using their UL values. This “UL hierarchy tree” should be treated as just one possible way of viewing the entries. It is useful in managing ULs and in inferring (sometimes incorrectly) some knowledge about an entry if the entry is not in the Registers or only a few details are available.
• The position of the UL of an entry in the UL hierarchy tree has absolutely no effect on the normative definition of the entry.
• The UL hierarchy tree can be built simply by looking at the UL of each entry and building the hierarchy: it does not matter whether each entry is a NODE or a LEAF during the construction of the tree. However, of course in most situations it is desirable for new entries to use ULs such that they result in sensible additions to the UL hierarchy tree (normally meaning no entries “under” leaves, etc).