Coats in car seat

Feel a bit stupid asking but I was wondering about babies wearing coats in car seats.

Ive heard and see that they shouldnt be wearing a coat in a car seat and the main reason is because if the coat is puffy it creates more space between the harness thus making it more dangerous.

Is it a general rule for all coats in general I have a few fluffy coats which I wont be putting her in while shes in the car seat but I have other coats that aren't as thick.

Are these still acceptable or completely opt out of wearing coats?