Stickies are usually important or commonly used threads in a specific forum. In this "Devil Fruit sign-ups" forum,
this would be the thread you're looking for. They'll have a ship icon in the box next to them, denoting their sticky status. You should try read those before posting in a specific forum, they're quite helpful
PC is short for "
Player (controlled)
Character". Pretty much, it's the status your character will have after it's approved. You'll be using your PC/character in multiple threads, interacting with other user's PCs/characters as you go, hopefully building up a nice story for yourself. PCs are individual to each user. I, for example, cannot control your PCs. In turn, you cannot control mine.
NPCs, on the other hand, are
Non Player Characters. These ones can technically be controlled by whoever feels the need to use them. These can be a character that a user doesn't use a whole lot, or a random group of marines that happen to be chasing your thieving pirate down the street. PCs are special since you can only make ten, while you don't have a limit of NPCs you're allowed to create.
So... In the case of your Devil Fruit (DF), it wouldn't exactly be nice to randomly make a massive crack in another user's PC for example. It's pretty likely that they'd end up dieing in a single hit, and other people don't exactly appreciate that. Attacks that kill PCs in a single hit unfairly won't be approved. We also don't want people cracking the planet cleanly in two, either.
Would you mind giving an example? Like how large of a crack would your average Joe be able to make on wood, or stone? How large of a crack could an incredibly strong person make on the same substance?
Feel free to PM me if you'd like me to elaborate on anything, or you have any more questions.