Sorry for the wait... this is explained in
avid.secure.force.com/pkb/articles/FAQ/Pro-Tools-Tiers
How are GrooveCell and SynthCell virtual instruments authorized?
They are authorized by iLok licenses. These licenses are tied to your Subscription or Software Updates + Support Pan. Avid began depositing these on Apr 22 (in advance of the release) and is expecting all deposits to be completed by the end of April. Please watch your iLok account for these licenses.