Imagine you create a particle class:

define class myParticle as custom
   x = 0
   y = 0
   z = 0
enddefine

Now whether those properties for position are against some absolute frame of refernce or a relative one is a big deal.

Because let us say the properties are for some sort of observer independent location. In that case any motion of the particles is absolute motion. In order to find the relative velocity, which you would need to do in order to calculate length contraction (the relativistic measurements we're going after), you could add and subtract the absolute velocities.

But here's the problem.

Light, assuming it is implemented as the photon particle, in this case will not have a constant relative velocity.

The choice of which co-ordinate system to use in this case makes a big difference as to how the rest of the program is written. That's why Einstein banished the absolute reference frame, because he had to make the speed of light constant in all relative frames.