“Bioavailability” of a Cannabidiol is how soluble it is in our blood stream and how much of it diffuses through membranes (by osmosis) and reaches the intended target.
Bioactivity is what happens when the Cannabidiol reaches its intended target.
Δ2-Cannabidiol has a functional response within live tissue. In the case of the bioactivity test, the binding affinity, or binding strength, of the CBD molecule to the CB2 receptor site is measured. CB2, cannabinoid receptor type 2, is a G protein-coupled receptor from the endocannabinoid receptor family. It is encoded by the CNR2 gene in humans. It is also called the “mimotope”. The antigen, CBD, has an epitope which binds to the corresponding mimotope.