A prescriptive label is calling something what you want it to be (regardless of if it is) often in the hope to make it that way.
e.g. Calling her your girlfriend after just one date isn’t gonna make it true, buddy.
A descriptive label is a term used to describe an already existing dynamic.
e.g. There wasn’t really a day that we started dating, after about six months we just realized that we were.
Prescriptive labels are ‘wishful thinking’. They’re bad because you’re assigning a value to something that doesn’t exist and asserting a level of pressure on the other person to conform to that ideal.
Descriptive labels tend to form organically and arise after everyone involved is comfortable using that term. Sometime you just start using them because that's what you are and have been for a while. Sometimes you have a conversation about what everyone's comfortable with and come to a consensus that way.
Why use a label at all? Labels exist because we use things like words to more efficiently communicate. You know that coffee cup sitting on your desk? Does it really only ever have coffee in it? Do you sometimes use it for water, too? I bet you do. Then why call it a coffee cup? Why use such a limiting label for something that could be used in so many diverse ways. Did you know you can put things like pens in those, too? You can’t even drink pens!
We use a label like “coffee cup” because that descriptor let’s us know what it generically looks like. It’s probably ceramic and has a handle. If I said tea cup you’d have probably envisioned a more demure vessel. Possibly one with a saucer.
Being human beings we understand that it's more efficient to communicate with word despite how imperfect they are. We also understand that labels do not always minutely encapsulate the entirely of an object or idea. We understand it’s simply a word used to communicate a general concept and that that concept is flexible, organic and may vary among different people or cultures.