I am not so sure: each plug-in listening for player chat commands is notified when a chat command is issued. And each plug-in will (presumably) check if it is a command it is interested in.
So, each plug-in recognising a "/select" command will do whatever that command means to it.
yes and is why i said two things will happen, but in the case of /select /selectarea /select area they all do the same thing, they bring up the area selecting tool oO