A Building Automation System (BAS, sometimes called a BMS) is the centralized network of direct digital controls (DDC) that automatically runs a commercial building's HVAC — scheduling equipment, holding setpoints, staging rooftop units and VAV boxes, and raising alarms when something drifts. It's the difference between a building managed from one screen and one where someone walks the roof flipping thermostats. In construction specs, controls live under CSI Division 25, Integrated Automation.
The complication is that a real building accumulates equipment from many manufacturers that don't natively talk to each other. The Niagara Framework from Tridium is the most common answer: a vendor-neutral software platform that normalizes data from "any manufacturer's" devices into one system, with over a million installations worldwide. Distech and Automated Logic (ALC) are other common commercial control lines.
For an owner, the practical signal is whether the contractor actually holds the controls credential the job needs. The programmer qualification is Tridium's Niagara 4 Technical Certification — a real, person-level certification — not to be confused with a "Niagara Compatibility Statement," which is just a product spec. Residential HVAC generalists essentially never carry BAS certifications, so a verifiable Niagara (or Distech/ALC) credential is one of the cleaner ways to tell a genuine commercial controls contractor from a company that simply says it "does commercial."