The number on a supplement label describes how much of a compound is in the capsule. How much of that reaches circulation and becomes available to tissue is a separate question with a separate answer.
Absorption is a multi-stage filter
A swallowed compound must survive stomach acid, dissolve in the intestinal contents, cross the gut wall and pass through the liver before it reaches general circulation.
Each stage removes a proportion. The fraction that completes the journey intact is what pharmacologists call bioavailability, expressed relative to the same amount given intravenously.
For some compounds that fraction is nearly complete. For others, particularly plant-derived molecules, the great majority is lost before anything reaches the blood.
The chemical form determines the route
Minerals are sold as salts and chelates, and these differ in how readily they dissolve and how they are handled by intestinal transporters.
Fat-soluble vitamins depend on bile and dietary fat for absorption, which is why taking them with a meal changes the amount that gets through.
Two products stating the same elemental amount can therefore deliver quite different quantities, which is why the form is often more informative than the number.
The liver removes much of what is absorbed
Blood leaving the intestine passes through the liver before entering general circulation. The liver treats many compounds as foreign and modifies them for excretion.
This first pass can remove most of an absorbed dose. Increasing the amount swallowed does not necessarily overcome it, because the enzymes are working on everything that arrives.
Formulations that pair a compound with an enzyme inhibitor work by slowing that removal, which is also the mechanism by which some supplements interfere with medicines.
Absorption is regulated, not fixed
For several nutrients, the body adjusts uptake according to its stores. Iron absorption falls when stores are adequate and rises when they are depleted.
This means the proportion absorbed from a given amount is not a constant, and figures measured in deficient volunteers do not transfer to replete ones.
It also means that larger amounts frequently produce diminishing returns, since the transporters involved become saturated.
Why blood levels are the harder question
Reaching circulation is not the same as reaching the tissue where a compound acts, and some are cleared from the blood within hours.
Studies measuring blood levels answer a narrower question than studies measuring an outcome, and the two do not always agree.
Interpreting any of this for a specific product or a specific person is a matter for a pharmacist or clinician rather than for label comparison.