Up to my knowledge, pD2 is a marker of affinity of drug towards specific receptor and thus used to describe potency of medicine, but I still have a little confusion about it.
The negative logarithm to base 10 of the EC50 of an agonist is called pEC50 or pD2 (old term). It indicates the potency of an agonist but not its efficacy, the higher pD2=small EC50, reflects higher potency of the agonist.