“ab initio” meaning “from start” is referred to models that in a way can fundamentally and precisely explain the phenomena happening in their domain. quantum mechanics is an ab initio model for explaining material properties since it is fundamental and precise; classical mechanics is another example with its domain being the interactions between bodies of low velocity and momentum(not too low of course or we would enter quantum realm).
classical electromagnetism can also taken to be an ab initio model for electromagnetic interactions between bodies (also for it to be a model it must include classical mechanics) and its basic principles are Coulomb’s law, Gauss’ law and Lorentz law and Maxwell equations.