At a university that superscores, the best sections from two sittings are combined. A student with 700 verbal and 650 math in May and 640 verbal and 730 math in October has a superscore of 700 plus 730.
The university decides
This is not a College Board practice. Each university decides whether to superscore and states it on its admissions page.
Where it is not applied, the total from a single sitting is what counts. Strategy therefore cannot be built without looking at the target list.
How it changes strategy
On a list where superscoring applies, a second sitting can be planned around one section: concentrate on it and accept the risk that the other comes out lower, because the lower one will not be used.
On a list where it does not apply, that strategy is dangerous: concentrating on one section while the other drops lowers the total.
The link to sending scores
Most institutions that superscore need the relevant sittings sent in order to compute it. A student who sends only one sitting cannot use the advantage.
Some institutions require the full history, in which case a weak sitting is also visible. The rule is on the institution's page and is read before scores are sent.
How much difference it makes
The real gain is usually smaller than assumed, because a student's section scores across two sittings tend to be close together.
Still, a few dozen points can matter for a file sitting at the edge of a band. The criterion is whether that gain is worth the cost and time of a second sitting.
The order of the decision
Build the target list first, then read which institutions superscore, then decide whether to sit again. The reverse order means preparing for a strategy that cannot be used.