The simplest thing I can say about it is that it's a proof of being a member of The XIII Order. Back then before we actually knew the Order existed (KH1FM Secret Ending, Deep Dive), Mickey, Riku and Roxas was labeled as the "unnumbered". Maybe that's the case, as camouflage? 'Cause if you think about it, isn't it weird that the Order don't give a damn to whatever Mickey and Riku is doing? If Sora just lazed around in KH2, I bet they would still say something like, "lookout! He's scheming/plotting something!", while Mickey was trying to find a way to contain Kingdom Hearts.
Correction is welcome.