Five ideas carry the whole product. Once they are clear, every panel in the app is a view of one of them.
A system is a set of components — coils, compressors, fans, pumps, packs, heaters, recovery devices — joined at ports. Every port carries one fluid (air, water or refrigerant) in one direction, and a connection joins an outlet to a matching inlet. That is all the solver ever sees: the 3D scene is a picture of the graph, and where you put a component on the floor changes nothing about how it performs.
A port left open is where the world enters. An open air inlet takes entering air (dry bulb and wet bulb — the weather, or a room's return); an open water inlet takes a supply temperature; a DX coil with open refrigerant ports takes a feed from a condensing unit drawn elsewhere. Everything else the solver works out for itself.
The Design tab solves the system at a design point — one set of entering conditions — and reports every component's in/out states, duties and power, the compressor's operating point and the totals. The Simulate tab does the same 8 760 times against an hourly weather year, with a controller deciding each hour what the machine should do. Nothing is estimated from rules of thumb: every component is a physical model built on published correlations and catalog data, and a solve that cannot converge is reported as an error rather than a guess.
Every system carries one category — rooftop, split, air handler, chiller, condensing unit, fan coil, evaporative, recovery, plant, unit cooler, or a label of your own. It is how systems are filtered in the Design tab's sources, in the public directory and in supplier offerings, and how a supplier says what they build. See Kinds of systems.
Any saved or published system can be placed into a design as one block — a nested system — that shows the open ports of the system inside it. Wire its ports like a component's, and the solver flattens the whole thing before solving. A nested system is a reference by default (the design follows its source when the source changes) and can be copied in or exploded into plain components at any time. This is how you combine a supplier's published chiller with a handler of your own, or build a plant from units you have already designed. Three levels deep; a composition may not contain itself.
On the Simulate tab a system becomes a block: a machine that serves rooms and is coupled to other blocks — a chiller feeding an air handler's coil, an exhaust unit feeding a recovery unit, a condensing unit feeding an indoor coil. Rooms ask for what they need each hour, the blocks answer in turn, and the building's comfort and energy fall out of that conversation. See Building simulation.