x1 (number; default: 1) is the physical speed supremum (for objects of positive masses)/celeritas/c0 expressed in units x2 (default: in terms of natural parameter c0; id est: unitless/dimensionless and equal to 1) in (paradigm/system of) physics x3 (default: this, our actual. physical universe and reality);
In the Newtonian model, for any finite nonzero units of spatial length and temporal duration, c0 is infinitely large. In the modern Relativistic/Einsteinian paradigm of physics, c0 is a finite positive number for any finite nonzero units of spatial length and temporal duration; thus, in such cases, we can set it equal to 1 (begin measuring in terms of percentage of c0) and gain scaled Relativistic results; by Maxwellian electromagnetism, c0 = 299792458 m/s. Notice the lack of reference to the speed of light and of media through which the wave/particle propagates. This is a physical constant of a universe; any entities of zero mass in a universe will propagate at c0 in an ideal perfect vacuum/classical free space - we just happen to name three such entities "light", "gravitation", and "the strong nuclear force". See also: plankexu, gravnutnoia, boltsemaku, avgadro, ocnerta.