If any of the forests being cleared are attached to the context database, then xdmp:forest-clear is run asynchronously, returning immediately but doing the clear in a background thread.

If the forest being cleared is not attached to the context database, it is run synchronously and will block until it returns.

The xdmp:forest-clear function is not transactional, and the best practice is to not call it in a transaction that is also doing updates.