Jumbo Driver is correct. In a nutshell, speeds within MNPS are referenced to Mach because "Mach No Technique" is employed for longitudinal separation on both Random Routes and the OTS.
So for ATC flight plan purposes, before entering / after leaving MNPS (the so-called "domestic" routings) speed is filed in knots TAS, even though one is in all probability still going to fly the same cruise Mach No as one would within Oceanic airspace.
Hope this helps