I think the distinction between possession and switching can exist, you just have to draw the lines in the sand. If you draw parallel lines, like we did, they won't overlap. Otherwise, they are very similar experiences, possibly on the same spectrum of experience.
When have what we call "sensory based" switching where the tulpa switched-in gets "connected" to the body. Once they are, they're essentially the default personality. When they zone out, they're still switched-in and in control of the body, even if they're distracted or doing something in wonderland. In our experience, we believe the switched-in headmate also processes more on the unconscious level than a headmate who is switched-out, even if they are talking to you.
Since we drew parallel lines, something I don't think many other people have done, these don't overlap. We switch with a ritual, otherwise if a headmate feels close to the front they always possess not switch. As a result, possession actually feels different than switching for us.