When you subscribe to DevNet Pro, you get:
- DRK Special Edition (Contains a selection of the extensions and components from Volumes 1 and 2)
- The current DRK (which is currently 3)
- The next 3 DRKs
So basically, DevNet Pro includes DRK 1 and 2, which were part of Studio MX plus, plus a subscription to DevNet Essentials. Hope that helps clarify the question.