A stage is where a task has got to — the columns work moves through. Each folder defines its own, so a folder can have exactly the steps that work really goes through rather than a generic to-do, doing, done.

Every stage is one of a few underlying kinds: open, in progress, review, paused, done, blocked or closed. That is what lets reports understand "finished" without caring what your folder happens to call it.

Stages and the timer are connected. Moving a task between stages prompts you to start, stop or restart timing, and restarting is usually what you want, because it records which stage the hours actually went into.