The relations for fields, tables and values have each one domain that is made up from a kind of integer numbers, i.e. strings of a few numbers often beginning with some zeroes. These aren't really numbers because obviously, there is a significance given to leading zeroes. Thus a five digit string denotes a field, a four digit string denotes a table, and a six digit string denotes a value. If we want to regard these digit strings as numbers we have to admit, that these numbers are not a unique classification of data items, but not more than a key to the relation, where otherwise would be no simple key. In fact the only usage of these keys is for the relation of tables, while fields and values have a composite key which is sufficient. In the latter cases, we won't make use of the digit strings.
Nevertheless, there is evidence that what was intended with the data item numbers is a kind of classification of data items of HL7. How else would this meaning that was given to the leading zeroes be explicable? It wold have been sufficient just to assign numbers which can not uniquely specify an HL7 data item but which do their job within a single relation. Anyway, we don't need a HL7 data classification here.