By looking previous detailed reactors types, we can define four ideal reactors (Batch, CSTR, Semibatch and PFR) generally used for large scales and just mentioned that real reactors are sometimes a combination of ideal reactors.
The first (Batch) is a simple scale-up laboratory battery where concentrations and temperature can change with time (high reactant concentration), so this is not steady state and can assumed to be a closed system.
The CSTR really quite different looks like a battery reactor where we can change reactants and removing materials and this change things little bit (adapt for low reactant concentration), that means its works in steady state ( ) and then Semibatch mean a batch reactor where for example it might like one component like A and then continuously feeding B, so it is not a steady state reaction. Another steady state reactor is the Plug Flow. So for the three first reactors, it’s assumed all uniform spatially, that means no concentration gradient and temperature gradient across the reactor. That not the case for the PFR, so the concentration is continuously changing as he move in the reactor.