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This is a classic question while interviewing engineer candidate. It is all about term, marketing strategy and any other business matter. If you look back the history, originally we have relay system then so on... so on... and now Sie**** sounding PCS, Rock**** sounding PAS and even they say about programmable automation controller (PAC) rather than say PLC.
No matter what is the name, DCS, PLC, PAC, PCS, PAS and any others... i think the good company management and engineer will look at their requirements in detail first then find out the best "automation system" match to their requirements. "The best" means money, continuous support, excellent service, etc.