When is something dissolved and undissolved in a solvent?

For example, I know that ionic compounds dissolve in polar water through hydration KA(s) <–> K+(aq) + A-(aq). Then I came across the definition of the dissociation constant ([K+][A-])/[KA]. But now I'm wondering what [KA] actually means, since concentration is defined as mol/l of dissolved substance, but KA is not dissolved, unlike K+ and A-. If I pour salt into 1L of water and at some point the solubility product is exceeded, salt precipitates. Does this precipitated salt still count as [NaCl]? If not, how do I distinguish whether a solid sediment is part of it (in visible quantities) or whether the invisible NaCl is already part of the equilibrium reaction?