While it may appear obvious, it is reiterated and stressed that the units of measure in each frame is invariant. Thus an Msec in M will always represent the same interval of time anywhere and anytime in M; the same goes for the Nsec in N, and any other arbitrary unit of measure Xunit in any other frame X. Note that this applies to any of the basic units of measures for time, distance, mass, charge, etc.

For those who are easily confused, this refers to the units of measure, and not to the actual quantities being measured. For example, a bound quark having less mass than a theoretically unbound quark presupposes a consistent unit of mass (or energy) is being used, without necessarily implying any invariance in the actual mass of the quark.

Note that for the contrary to be true, that the units of measure are not invariant, means that our all observations, measurements, and experiments, become irrelevant, for we lose a standard by which to compare one quantity against another.