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:
DocBook
JATS (Journal Article Tag Suite)
TEI (Text Encoding Initiative)
STS (Standards Tag Suite)
BITS (Book Interchange Tag Suite)
DITA (Darwin Information Typing Architecture, which includes a tag set and a lot more)
Akoma Ntoso (an expandable tagset)
NIEM (National Information Exchange Model)
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.