UMLR — Decide whether the document divisions are "sub clauses" or "subclauses"

The spec uses both.
The spelling with the space predominates.

ISO probably has a preference. Their document How to write standards http://www.iso.org/iso/how-to-write-standards.pdf use "subclauses" as one word.

However some of their actual documents use either "subclauses" or "sub-clauses". I didn't see anyone using "sub clauses" and their documents appear to be internally consistent.