They claim it's good for heartburn. It's the calcium carbonate, although that's described as an inactive ingredient. That's how Pepto Bismol functions as an antacid. IIRC, Bismuth subsalicylate sounds like a salt of a weak organic acid, i.e. salicylic acid. Acetylsalicylic acid, aka ASA, is aspirin. Hence the warnings about Reye's syndrome.
Calcium is the major ingredient in most OTC antacids.
Interesting side note on Pepto (and store-brand quivalents). Bismuth subsalicylate, one of the major ingredients, is about the only 'medication' that is effective (in high doses -- 6 tablets or 6 t-spoons per day for 21 days with an additional complement of antibiotics) against the Helicobacter pylori (H. Pylori) bacteria that causes a stomach ulcers.