Sometimes an adjective describes its referent: “My wonderful brother” means I think my brother is wonderful.
Sometimes an adjective distinguishes its referent from others like it: “My younger brother” picks out which of my brothers I’m talking about.
Sometimes people (jokingly or seriously) confuse these uses; I might say “My wonderful brother” to talk about one of them, and the other might joke “What, as opposed to your awful one?”
Does this make sense? Is there formal linguistics vocabulary for talking about this distinction?
my wonderful brother
“wonderful” is a descriptive adjective.
my younger brother
“younger” is classifying.
Depending on the context and the intention of the speaker, typically descriptive adjectives can be used to classify (as in the joke you mentioned: “my wonderful brother” as oppossed to “my awful brother”).