I completely overlooked the response here. Sorry for that.
It doesn't feel like one because we didn't talk about it. It was just a pattern that was used right from the start. We switch every night and that's what I need and they as well because it comes closest to being as much with the other as possible. There aren't boundaries around that because our needs were met right from the start and no one would think of changing anything around our sleeping arrangement. It's kind of like the most natural need for sleep, if this makes sense. I need to get to bed every night, I want to sleep next to my beloved ones, but they don't sleep in one bed. That's why I split my time as equally as possible.
Maybe the word boundary isn't stored in the right corner of my dictionary. In respect to poly it is something for which you sit down and discuss with your partner. Something like that never happened.
I asked Sward, who is with me at the moment, if we ever did something like defining boundaries. He pondered about that for a bit and said: "Well there are some. My knife, my cup, ..." and laughed. After that he said that it all came naturally. Along the lines of: What would I like to have, what are the others able to give? But none of us has the impression of some kind of "rules" being present in our everyday life. It's just how you work in every relationship: Look out for the other person(s) involved and work with that.
Facts: 30, female, bi, v-type relationship with Sward (husband, straight, mono) and Lin (boyfriend, straight, mono), poly-fi and co-primary.