This is what I found on the Avaya docs:
Software licensing for IP Office R9.1 will remain predominantly in ADI, however there will be instances where PLDS licensing will be used. For new IP Office R9.1 quotes, the ASD and A1S configuration tools will default on licensing in the following manner:
o ADI Licensing Scenarios
Most Basic/Essential/Preferred/Server Edition deployments (same as R9.0)
o PLDS Licensing Scenarios
Essential and Preferred Edition in an Aura branch deployment (same as R9.0)
Preferred and Server Edition when quoted with either IP Office Contact Center or Avaya Contact Center Select (new to R9.1)
All IP Office Select scenarios (new to R9.1)
o In addition, there will be an option to configure any new Basic, Essential, Preferred, or Server Edition with PLDS licensing. (new to R9.1).
o Please note that existing systems upgrading to R9.1 should purchase the upgrade license in the same license format that resides on their system.