☆ Mark sits at the top of every question. Press it and it fills in — ★ Marked. Press it again to unmark.
It is not just for this session
A mark belongs to you and that question, for this exam, permanently. It is still there next week, in a different session, on a different device. That is what makes it useful: Marked is one of the four question pools in Build your test, so you can come back and drill only the ones you flagged.
What to mark
A question you got right but guessed.
One you got right for the wrong reason.
One whose explanation you want to re-read when you are less tired.
A rule you know you will forget.
You do not need to mark the ones you got wrong. Those are collected for you automatically — see "Missed questions".
In the results
The end-of-session review filters by All, Incorrect, Marked, Revealed and Unused, each with its count. You can mark and unmark from that list too, so a review pass is also a triage pass.
Marked is not the same as missed
Two different lists, deliberately:
Marked is what you chose to flag.
Missed questions is every question you have ever got wrong, gathered without you having to do anything.
Use marking for judgement calls the app cannot make for you.
