[icegreentea]

I think its a reference to state space control. Traditionally, PID controllers fell under 'classical control', which is basically the entirety of control theory before the rise cheap computing. They involved devising all sorts of "tricks" to make designing controllers tractable by hand (more or less) - stuff like frequency domain representation, pole-zero plots, root locus method.

State-space representation is a strictly time domain approach, where you break your model and controller into a bunch of 1st degree DEs. This method is really only tractable with computing support.

In general, state-space and frequency domain models can in theory accomplish the same goals. You could for many situations design a PID controller using classical methods, and then take the same situation and use a state space approach, and in the end result in the equivalent controller/behavior.