Straight answers
How does Mark for Review work on the digital SAT?
Mark for Review is a flag you place on a question to find it again later. A review screen, reachable from the question menu and shown at the end of each module, lists every question and which ones you flagged, and you can jump back to them — but only until you submit that module, after which it is locked.
Flagging does not change your answer or your score. It is purely a bookmark for you. Select your best answer first, then flag, so that if time runs out the question still has a response in it.
The best use is triage. Answer what you can quickly, flag the ones that need more thought, and use the review screen near the end of the module to spend your remaining time on the flagged ones, in whatever order you like. Within the module you are in, you can move freely between questions.
The limit that matters: you cannot go back to a module after you submit it. That is why flagging is a within-module tool. Do not plan to fix a Reading and Writing question during the Math section, because that module is already closed.
Watch the clock as you triage. A flagged question is a promise to yourself to come back, and that promise has a cost. If you flag many questions you will need time at the end, so build the habit of flagging only genuine coin-flips and moving on from the rest.
Practice it in Bluebook's official practice tests so the review screen is familiar. In Brainrise drills, try the same discipline — answer, flag only if needed, and keep moving — so it is automatic when the clock is real.