Imagination, hallucination. Hallucination would always be your imagination, because it's not real. But not everything you imagine is a hallucination, because a hallucination would have to feel pretty real. Like, I dunno, if I spent time with the tulpa and he touched me, I might imagine the feel of his touch. Perhaps touch is one of those senses that is very close to the blurred line for me, as a just "imagined" touch already feels very "real", but then there's those times when he grabs me and I just go all. Woah, something ouched me. Like someone really did touch me and it could fool me into thinking it was another person, except there's no one else there but the tupper.
Feel was my example sense, but it could be anything. Sight, hearing, whatever. Those are the ones people usually want, but you can get to the whole hallucination part by getting good at imagining whatever sense it is you want to become a hallucination. And accepting the hallucination to be there, so it doesn't go away.