Jwscud
VD / MD is the Design Dive Speed
VC / MC is the Design Cruise Speed
Both used in flight testing for certification process (CS-25)
From the FAA Type Certificate Data Sheet of B767:
VD = 420 KCAS to 17,854 ft/.91M above 23,000 ft, linear variation between these points.VFC = 390 KCAS to 17,600 ft/382 KCAS at 23,000 ft/.87M above 26,000 ft, linear variation between these points.