They used to have skill points. when you ranked up, you get a few and you can use it to get weapons and skills like faster reloads or farther knifing distance. Usually a skill may be 1 or 2 points, an attachment may be 2 or 3, and a weapon may be around 5
Now they got rid of that (sorta) Now you earn currency from matches so you dont have to wait to rank up to earn points and you can get appearal and weaps/ attachments and the skill points you had were reset so you could re assign them so now the skill points are used for skills/ perks.
I kinda liked it because its not easy leveling up on this game