F4F

F4F is a thing some scratchers do (standing for: Follow 4 Follow). It means that if someone follows the scratcher, they will follow them. F4F is mainly only popular with smaller scratchers. Bigger scratchers like Griffpatch do not do F4F, mainly because that would take a lot of work due to the amount of followers they gain in one day. Though it is mostly big scratchers, there are many small scratchers that oppose F4F and will not do it. The reasoning that the smaller scratchers don't do F4F is probably for the same reasons as big scratchers.

S4S
S4S is commonly used on Instagram. It is when two people shout out each other on their profiles and encourage their followers to follow them. It is not popular on scratch though.

UF4UF
UF4UF was a policy invented by NinjaJacksonFi (standing for: Unfollow 4 Unfollow). It basically means that if someone is following you, and you do F4F, and they unfollow you, you will unfollow them.