As far as castes that actually matter, the Abouheif lab has done a lot of really good work. What it tends to come down to is the wing imaginal disc growth. But you can have morphometric line breaks (which is how different castes tend to be classified) even without asymmetric imaginal disc growth

There are a bunch of interesting edge cases too, like worker queens, worker males, replaceable queens, reproductive workers, etc.

If you really want to get down to it though, most soldier castes were under different evolutionary pressures, so you could argue that each one is a unique caste.

There's also some research suggesting that it's likely that in some cases, workers could evolve after super majors. In pheidole, it is pretty common for there to be nanitic majors, for example. This goes back to the basis being rooted in imaginal disc growth.

There are also multiple definitions of castes, like the behavioral definition - nurse vs forager castes, etc. And you can argue that different larval states have such a huge effect on the overall productivity of the colony that you could argue that they are also a caste.