Serving size (units per serving)
The amount a label treats as one serving, which for capsules, tablets, softgels and gummies is a count of units -- often more than one. Every "per serving" amount on a Supplement Facts panel describes that whole serving, not a single capsule, so a "400 mg" line on a four-capsule serving is 100 mg per capsule.
Example
A label listing 400 mg of magnesium per serving where the serving is 4 capsules delivers 100 mg per capsule; taking one capsule delivers a quarter of the listed amount.