Linux Filesystem Overlay - what is workdir used for? (OverlayFS)

The workdir option is required, and used to prepare files before they are switched to the overlay destination in an atomic action (the workdir needs to be on the same filesystem as the upperdir).

Source: http://windsock.io/the-overlay-filesystem/

I would hazard a guess that "the overlay destination" means upperdir.

So... certain files (maybe "whiteout" files?) are non-atomically created and configured in workdir and then atomically moved into upperdir.