I don't remember seeing an "m" before . My guess would be that it stands for "measure", i.e., the magnitude of the angle as opposed to the angle itself as a geometric figure.
To add, when we take a sum of two angles, we must use to convert angles as figures to numbers first and then to add the results. That's why is used in . To say that two angles are congruent, we don't need since we could say . The latter fact is equivalent to .