Sorry, I'm trying to do too much at once and doing it all badly...

I meant to mention that the explanation doesn't explicitly say anything
about what a "leaf" is, but it gives a good overview that should make what a leaf is clearer.  A leaf is a good old computer science bottom-of-the-tree node.
It's derived from a basis, and has no descendants.  Branches and leaves in a
distributed scm are just side effects a lot of the time; it's up to the
participants to merge them back into a basis.