I recently sat ATPL PoF exam. Mcrit is the point at which airflow over the aircraft at some point reaches speed of sound. Operationally its on a day to day basis of not much significance. The important limits are Vmo and Mmo. Hence the change from constant IAS to constant Mach in the climb around FL260 and vice versa on a descent.
737 max cruise is about 0.74 as above this you get problems if the mach trimmer fails (got this from the IAS/TAS discussion (see my excellent post!)) earlier on the Tech log section.