ftp,
You already have me stumped with Mdr !
Mcrit is formally defined as the Mach number where the airflow somewhere locally reaches the speed of sound.
The "dr" sounds like "drag rise", which would make Mdr the Mach number at which the additional drag due to Mach effects becomes measurable (1%? 5%?), which is usually somewhere above Mcrit. Also known as the "drag divergence" Mach number.
So I would be curious about Mdi (or Mde?) too !
CJ