As far as I know, they generally mean the starting concentration of the acid and base, but in this case Ca can mean the total starting concentration of all sources of the anion PO43- and Cb can be the total starting concentration of all of the sources the cation Na+. That doesn't really help, since we still don't know how to work out what these starting concentrations are individually.
I'm adding a certain amount of NaOH to pure H3PO4 - how do I figure out how much of each salt is produced? I can see now that Ca genuinely does refer to the starting concentration of the acid because any new salt formed capable of producing that anion can only add up to the same starting concentration. Meanwhile, I will take it without question that Cb retains the definition of the starting concentration of the base NaOH.
So essentially, when an acid (or multiple acids) is mixed with a base (or multiple bases), the salt(s) formed do not affect the pH of the solution; it can be calculated directly from the original starting concentrations of acid and base, as shown by the equation in the pH calculator used by ChemBuddy (as long as the new Ca and Cb in the solution formed by combining the acidic solution with the basic one is calculated from the previous Ca and Va, and Cb and Vb). Am I right?
That still doesn't solve the main part of this problem, though. I can find [H3O+] and [OH-] from the starting concentrations of acid and base, but how do I find Na3PO4, Na2HPO4, NaH2PO4, H3PO4, H2PO4-, HPO42-, and PO43-? I'd like to learn what's going on in terms of the equilibria (as I need to calculate these concentrations exactly rather than with approximations).