Could someone teach me a little about what common5 and common3 are?

[Rophlecktuine]

Or is there a guide that might explain a bit more?

I notice whenever I download a PSA of a character (Like sephiroth for marth) it comes with a common 3 file.
Ive already tried using one before alongside the common5 and it caused the game to freeze.

Do I remove common5 and replace it?
Will this cause other problems for me in the game?

Thanks for any advice guys!

[ABloodyCanadian]

Common3 is where all the items are placed, Common5 is the Character Select Screen and the Stage Select Screen in 1 file.