I believe that "stand alone" means not connected to other systems. Since you don't have DP (you need to buy additional module) or MPI on ET200S CPU can be considered not to be suitable for connection to the other systems.
You can use it if you don't need some special modules which doesn't exist for ET200S (I guess there are such), if processing speed and memory resources are satisfactory, and most important if someone (ie. customer
) is against its usage.
Thanks #bosandzeros, for your kind reply
...But 6ES7 151-7FA20-0AB0 has the MPI+DP I/O and it can be used in hazardous situations also......Can you elaborate the term 'Special modules'..In this case we don't need safety module also......Thanks in advance....