"Multihack" is a catch-all term for things that don't fit under "Wallhack", "Aimbot", "MD5 Tool", or "Gamehack". It generally means a hack that does more than one thing, but not always. The Number is likely far more meaningful internally, and is a sort of 'signature' of the application/tool/cheat that was caught violating Punkbuster's policies.