# What is the difference between the choice operator and nondeterminism in CSP?

I'm learning CSP now for my research. First time doing formal work in CS. I'm having a hard time understanding the difference between choice $$\square$$ operator and the nondeterminism $$\sqcap$$ operator. Does the former perform a completely deterministic choice only based on the inputs from the environment, while the latter can make a nondeterministic choice arbitrarily?

My book says that $$(a \rightarrow P) \space \square \space Stop$$ is not the same thing as $$(a \rightarrow P) \space \sqcap \space Stop$$, because the former is the same as $$P$$, but I don't see how that is if $$P$$ cannot decide to stop based on an input.