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 still use the lower-absorption plain form.