Originally Posted by SchrodingersCat
"prescriptive" secondary basically means you're "not allowed" to ever become more than a secondary, you'll never be as valued as the primary, and if your relationship ever becomes "too threatening" to the primary relationship, you're out the door.
Compare to "descriptive" secondary, where it's more like you just happen to have your life not as entangled (i.e. your partner has a wife with kids together, shared bills, and a joint mortgage) so the "secondary" status just describes the nature of your current relationship, without forcing it in a box that says it will never be allowed to become something more.
thought this would be worth repeating here.