Specifically, it’s DND 3.5e with some changes.
So, you have the best full version of DND (3.0) with all the changes made to make the system function the way it was intended and be much more balanced (3.5), with another total revision to balance, the introduction of many new mechanics, over a decade of new content, and an active publisher who makes massive use of the OGL (PF 1.0).
This sure is a take.