Systems interact through inputs and outputs, right? I mean, one system's output can be another system's input and even the second system's output can be the fir

Systems interact through inputs and outputs, right? I mean, one system's output can be another system's input and even the second system's output can be the first one's input. I guess my question was if there's something that can't be described or modelled with graphs. As I said, my intuition is fuzzy, because I don't know much graph theory.