The [`arc diff` userguide](https://secure.phabricator.com/book/phabricator/article/arcanist_diff/) says "you can create just a diff (rather than a revision)", but so far I have been unable to figure out the difference between both.
I looked for example at the [Differential userguide](https://secure.phabricator.com/book/phabricator/article/differential/) or the [explanation of the differences between review and audit](https://secure.phabricator.com/book/phabricator/article/reviews_vs_audit/) and of course the [Audit userguide](https://secure.phabricator.com/book/phabricator/article/audit/).