[Yen]

I didn't bother to read anything, so I'm just throwing in some (hopefully) related information.
On my server, I reserve attr[2] to attr[5] of every NPC for specific things.
attr[4] is the NPC's identifier; it's a value based on the NPC's index. If I want to refer the serverside to a specific NPC found on the clientside, it uses this to figure out which NPC I want.
It's basically a way for me to pass a reference to this NPC from the clientside to the serverside, or vice-versa.