Pure Stock is a muscle car class. Not NHRA. It's related to the F.A.S.T. racing and usually held in concert with them. I'm not up on the rules (more so for FAST class racing) but it's basically the hobbyist take on NHRA's Stock Eliminator.
Been to a few of those races at Stanton Mi. The pure stock cars sure run a bunch faster than my stock musclecars did over the last 40 years. I took my Charger 500 once.
Ive got an AAR Cuda that I am going to return to mostly stock and plan to run pure stock. Maybe F.A.S.T. Ill either be a quick pure stock or a slower FAST racer.