What are the Veracity definitions of head, leaf, and branch?

Veracity branches are more git-like than hg-like: they're essentially pointers to changesets. They mostly move on their own as you'd expect, but it's also possible to manually move them around. (Veracity also stores the history of branch names, so it knows which changesets have ever belonged to a branch, allowing some hg-like branch behaviors.)

Veracity leaves are the leaves of the version control DAG: nodes with no descendants. Leaves are leaves whether or not there's a named branch involved. 'vv leaves' returns all of these.

A Veracity head is a changeset that a branch currently points to. Usually a head is also a leaf, but not always. 'vv heads' returns all of these. 'vv heads -b <branch>' returns all the heads for <branch>. (The heads command doesn't support a --max argument.)

'vv parents' will tell you the parent(s) of the new changeset you'd create by committing. Usually there's just one, but if you're merging, there's two.

'vv update -b <branch>' will do nothing if the branch is ambiguous. If it resolves to exactly one changeset, it will update the working copy to that changeset. Otherwise, Veracity will force you to specify a changeset in a non-ambiguous way. The same is true when you type just 'vv update' while attached to a branch: nothing happens if the branch is currently ambiguous.

'vv log -b <branch>' will start at the current head(s) of <branch> and walk the DAG, showing info for each changeset encountered. 'vv log -b <branch> --max 1' will show only the first head of <branch>, but it's not really relevant to the behavior of update, as described above.