Coupling. A join between two blocks at their open ports — water, air or refrigerant — carrying last hour's state across, one hour behind. Couplings between blocks.
A building is rarely one machine. A chiller makes water for three air handlers; a condensing unit on the roof feeds an indoor coil; an exhaust unit hands its air to a recovery unit. A coupling is that fact declared port by port on the building: this block's water outlet feeds that block's coil inlet, this coil's return goes back to that evaporator. The receiving block then sees its partner's actual leaving water, flow and all, rather than a fixed boundary.
Blocks step in order and the return path runs one hour behind, which on a water loop is a ±0.3 K limit cycle — measured, and left as it is, since a cross-block fixed point buys half a percent. A block can serve water instead of rooms, holding a leaving-water setpoint for its partners; several receivers on one port are a manifold; refrigerant pairs put a condensing unit and its indoor unit in two blocks with the circuit split between them.
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 · Priority · Category · Nested system · Design point and TMY year