Bioavailability
The proportion of a declared dose that actually gets absorbed into the bloodstream in a usable form. Different chemical forms (salts, extracts, formulations) of the same compound can have very different bioavailability even at an identical declared dose -- the whole reason this site tracks "form," not just "amount."
Example
A 2011 randomized trial found a curcumin phytosome formulation raised total curcuminoid absorption roughly 29-fold compared to the same unformulated curcuminoid mixture -- yet 98.1% of on-market curcumin products with a matched form (1,962 of 1,999) are plain curcuminoids or an unspecified curcumin extract; only 1.9% name a phytosome-type formulation.