A 'RID' is an ID of a character who caused the NPC script to run, as has been
explained above in the introduction section. Quite a bit of commands want a RID
to work, since they wouldn't know where to send information otherwise. And in
quite a few cases the script gets invoked with a RID of zero (like through
OnTime special labels). If an NPC script needs this, it can attach a specified
character's id to itself. by calling the 'attachrid' function.

'attachrid' returns 1 if the character was found online and 0 if it wasn't.

This could also be used, while running in a script invoked by a character
through talking to an NPC, to mess with other characters.
Detaching the RID will make the RID of the script zero.