Dialing in a bell hsg is checking, and correcting if necessary, how well the stock dowel pins center the bell housing's opening relative to the crank centerline before the trans is installed.
Here's how you do it:
https://www.moderndriveline.com/prop...ment-indexing/
Here's how you fix it if you find a problem:
https://www.summitracing.com/search/...20dowel%20pins
Most do not do this - and most won't have a problem if they don't - but it is a good safeguard against mass production tolerances, casting defects, etc.