I would put -5 at not knowing how to play much at all. The first timer, or the guy that played with his brother a couple times or that classic guy that played a little back at school. Also that casual player that plays in a game every couple months., but what does -5 look like as a metric of play?
-4 Are the players with no interest in the rules. They have been playing the game for years, and don't even know most of the basic rules. Casual Gamers and Social Gamers.
-3 Know a couple rules, but not too many. They just want to have random fun. They don't want to think to much about a "silly game".
-2 They know most of the rules players might need to know.....but they are all twisted and confused. Often from a past Buddy DM with very Easy Button house rules . That they think are official rules. And they don't understand most rules anyway, and get confused by big words and concepts.
-1 knows the rules slightly below average, but they are stubbornly 'stuck' on something. Often a race or class, but just as often a play style. They often like to take odd feats, classes, abilties or whatever as the "like them" , and they don't help the character.
What can I say, I'm Deep.This feels way too far off the deep end in an otherwise polite conversation.
The fact is optimizers must play in games zero or below to have fun. They must play in a game that has little to no optimization. And they need an Easy Buddy DM.