How a superscore is built
Your SAT total is the sum of two section scores, Reading and Writing from 200 to 800 and Math from 200 to 800. A superscore takes the best of each across every sitting you send and adds them together.
So if you scored 690 Reading and Writing and 660 Math in October, then 640 Reading and Writing and 730 Math in March, a school that superscores sees 690 plus 730, which is 1420. Neither individual sitting produced that total.
College Board does not calculate this for you or print it on your score report. The admissions office does it on their side, using the reports you sent them.
What is the mean (average) of the following data set: 6, 16, 18, 23, 27?
Which schools do it
Many do, including a large share of selective private universities, but it is not universal and it is not something to assume. Some schools consider only your single highest sitting. A few ask to see every score you have ever earned.
The place to check is the college's own admissions or testing policy page, for the year you are applying. Search the college name plus superscore or plus testing policy. Do not rely on a list compiled somewhere else, because these policies get revised between cycles.
Make a column in your college spreadsheet for this. It changes which test dates are worth taking, so it is worth knowing early rather than in October of senior year.
How it changes retake strategy
If your schools superscore, a retake is close to risk free. A worse section on the new date does not delete the better one from the old date, so the downside of sitting again is the fee, the Saturday, and the preparation time.
It also means you can prepare one section at a time. If your Math is where the gap is, spend the whole block on Math and accept that Reading and Writing might drift down a little on the retake. Under superscoring that drift does not cost you anything.
If your schools do not superscore, the calculation flips. You need one sitting where both sections go well simultaneously, which argues for keeping both sections warm rather than specializing.
Score Choice and what you send
Score Choice lets you decide which test dates to report to a college. Most schools accept that. A minority require every sitting, and if a school does, superscoring usually still applies, so sending a weaker date is not the problem it might feel like.
Sending scores costs a fee per report beyond the free ones included with registration, and fee waivers cover score sends for eligible students. Fee amounts change, so check collegeboard.org for the current numbers rather than trusting a figure in an article.
Cancelling a score is almost always worse than keeping it. A score you keep but never send costs you nothing, while a cancelled score is gone even if it would have contributed a strong section to a superscore.
Superscoring across tests
Superscoring happens within a test, not across tests. No college combines an SAT Math score with an ACT English score. If you have taken both, they are evaluated as two separate results and you submit whichever is stronger.
PSAT scores are never part of a superscore. They are not reported to colleges as part of your application at all.