The species makes an acid version for each cannabinoid –CBDa, THCa, CBGa, etc. Acid forms naturally convert over time into active cannabinoids, or can be quickly converted in high heat conditions (typical industry method) –a process called decarboxylation.
There is some loss during decarboxylation. The formula for calculating total potential THC is:  THC + (THCa*.877)=delta 9 THC (Total potential THC).
Again, THC (or Delta 9 THC) refers to the cannabinoid post-Decarboxylation, or total potential THC.