The description of the element extent adresses only texts but not objects. It might be good to add to the description this other meaning as extent is used e.g. within supportDesc, describing there the number of leaves/pages of a carrier.
Proposal: "'extent' describes the approximate size of a text or of an object, e.g. as stored on some carrier medium, whether digital or non-digital or as number of leaves or pages. In either case the size is specified in any convenient units."