Category. What a system is — rooftop, split, air handler, chiller, condensing unit, fan coil, evaporative, recovery, plant, unit cooler, or a label of your own. Kinds of systems.
Every system carries one word saying what kind of machine it is — air handler, rooftop, split, chiller, condensing unit, fan coil, evaporative cooler, recovery unit or DOAS, plant, unit cooler — or a custom label. It is what the public directory filters on, what a supplier declares they build, and since categories entered quote matching it is part of how a request finds the suppliers who make that kind of thing.
The vocabulary is fixed on purpose and small: ten words an engineer would use, translated in the app and on the public site, with room for a custom label where none fits. A supplier who declares no categories is matched on capabilities alone; one who declares them is matched only on requests of those kinds — the same shape as the refrigerant rule, which also excludes only when both sides have spoken.
Open the builder — the examples in the Design tab's sources list include a machine of this kind, and the Simulate tab runs it against a year of weather.
DOAS — dedicated outside-air system · SZVAV — single-zone variable air volume · VAV terminal box · Economiser · Waterside economiser · Demand-controlled ventilation · Face-and-bypass · Head-pressure control · Humidity ceiling and floor · Cycling and modulation · Coupling · Priority · Nested system · Design point and TMY year