Originally Posted by KyleKat
There was a discussion on the forums a while back (I'm on my phone so searching for it isn't an option) that discussed the roles of secondaries vs primaries. Basically, what I took away was that a primary was someone you lived with and shared responsibilities such as chores, bills, taking care of any kids, etc. Secondaries, on the other hand, were more like friends or someone you were dating but you weren't super serious with yet. You'll do stuff like to out with them, pay for their meals, whatever, but you wouldn't pay their house or rent payment for them.
That is just some people's general description of the roles, but it isn't really a hard and fast way to determine who is primary and who is secondary. Plenty of secondaries contribute to household bills and chores.