Should be a simple one. Extent is so far a bare element offering no other descriptive possibility than a plain text description of the extent. My use case is the description of a corpus (TEICorpus or biblFull) in terms of number units (lemmas, tokens, etc.) so that it can be used as reference when indicating a frequency for a lexical entry. Typically [extent unit="token" quantity="2.3e6"/] for a 2.3 million token corpus.