It doesn't "decide" to do anything. Discounting the utility of describing an unspeakably great swarth of jostling particles with one word is foolish though. I

It doesn't "decide" to do anything. Discounting the utility of describing an unspeakably great swarth of jostling particles with one word is foolish though. It's effective to discuss large segments of reality without having to know the position and momentum etc of every particle, assuming those segments have somewhat well-defined and known behaviour.