Marcus, the terms are common in poly circles, not just here on this board (it sounds like you're saying it is "our" terminology here). Personally, I eschew the whole idea of such hierarchies in love relationships. Read some of SchrodingersCat
's posts on the subject - I like her outlook. She calls her approach "relationship triage," meaning that there is no automatic hierarchy in her relationships, as she responds to who needs her attention and time the most (I hope I got that right).
We have a long "Master Thread" on this topic. Maybe you will find some good, interesting info in this one:
Primary/Secondary: Merged Threads, General Discussion / Debate
(If you set your viewing preferences in your user CP to 40 posts per page, this long thread is only 6 pages. Much easier to read longer threads this way!)