We don't have limits per se, but we have expressed preferences before. Not rules, but a discussion of what makes us uncomfortable or needs/wants we have.
We do use the term secondary, but in a descripter sense. Just an expression of how much they're involved in our day to day life.
Me: 48 - Married, straight, male
Shiela: My wife.
Suzanne: My FWB
Adam: Shiela's LDR