I do use the terms primary and secondary. I also tend to get involved with my partners in a D/s familial dynamic. So while the heads of the family may be primary to each other and I am secondary to them, they as a unit can be primary to me. Very confusing to those not in the relationship sometimes.
I will also use it as 1WickedLush does and use it to indicate levels of entanglement on a legal level, but again as he says various levels of entanglement can occur with many partners so the term gets a little fluid in that regard as well.