We call quantum physics an ab initio model because the starting point (the Schrödinger equation) of the model cannot be further reduced to more fundamental concepts. This also applies to Classical mechanics, where the starting point are Newtons equations of motion, which cannot be further reduced within the language of the natural phenomena they describe.

In the same way, classical electromagnetism is an an ab initio model, because we have Maxwell’s equations and Coulomb’s Law that are the starting points which cannot be further reduced (if we remain classical).