Signals are the events Heidi notices and acts on: a job finished, a metric moved, a customer showed a warning sign, a message needs judgment. Outputs re-enter as signals, which is how work chains into workflows.
The loop is signal to decision to task to interaction: something happens, Heidi decides what follows deterministically, work gets scheduled, and a human hears about it exactly when judgment is needed.
That loop is why Heidi can run whole workflows rather than isolated requests.