These are storage volumes (hard drives) that are suitable for playback and have been designated in the Workspace browser as Record and Playback (R) or Playback Only (P) of media files in a project.

These are volumes that are not supported for media playback in Pro Tools | First (such as shared network volumes or CD-ROMs), or storage volumes (hard drives) that have been designated in the Workspace browser as Transfer (T) volumes. Transfer volumes cannot be used to record or play back media in a Pro Tools project. Designated Transfer (T) volumes can be useful for transferring project and media files between different systems.