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.