a collared ivory joint is like any collared joint, be it metal or phenolic,,,,that is, it's basically a cylinder that is placed over the wood at the joint, with the wood exposed. as in,,,take a look at your shaft. you have a collar made of some material and the shaftwood is exposed.
if "capped" joint means anything like a capped ferrule, the wood is not exposed. the material (ivory, in this case) covers the wood. it fits like a cap,,,,,like joint protector-style. if it doesn't mean that, the only thing i can think of is that it means a "full" ivory joint. in others words the joint is one solid piece of ivory, and there is no wood core.
whatever feels good to you is what's best.