When I first started in the HVAC industry working in a design consulting engineering firm, I came to accept that the HVAC sequence of operation and its associated contract specification, Automatic Temperature Controls (ATC), were provided by an ATC sales engineer. I’d go so far as to say this was the industry standard for designing HVAC systems. Heck, you could get some very knowledgeable sales engineer to come into your office and you’d explain the job and he (never met a woman sales engineer back in the 1960s) would help you out.
Of course, that section of the specification would state, “based on Honeywell Controls” or some other sales engineer’s company. This didn’t assure him he’d get the job, but many contractors would try and buy out the controls using the “base” ATC manufacturer.