What is an XML Tag Set?

I define XML Tag Set as an XML vocabulary or a domain-specific set of elements; such a tag set is a named collection of XML elements and attributes that describe at least one document type.

Some popular examples include:

Notice that I have defined this as a vocabulary not as a language. Vocabularies are descriptive rather than operational. XML vocabularies (tag sets) identify document components and data structurally and semantically. By this definition HTML is a tag set, but XSLT (which is also a set of tags) is not.

Further, when I talk about documentation, I mean textual and graphic material for reading by human agents. Machines, programming languages, and artificial intelligent agents need very different documentation.