All of the UBL fragments make reference to the documentation namespace for CCTS components:
The documentation constructs are not validated, the namespace exists only to distinguish the documentation constructs from the XSD constructs in the schema expression.
For some reason, UBL 2.0, and subsequently UBL 2.1, includes a schema fragment with this as its target namespace. This schema fragment has no declarations. This schema fragment is not imported into any other schema fragment. It is not used for any validation.
I've not included it in the draft for UBL 2.2, as I propose we remove it as it has no function. Although no-one has asked me why we included it in the earlier distributions, I cannot defend its inclusion then or in future distributions.
Please comment if you disagree and I'll put it back in.
. . . . . . Ken