Is a reset schedule always a cascade?

No. It is a cascade only if the value being reset is itself the setpoint of a real, independently closed inner loop with its own sensor feedback. If the reset computation feeds straight through to a valve or damper position with no inner sensor loop at all, that is feedforward or open-loop scheduling, not cascade control — the giveaway is whether an inner feedback loop with its own error calculation actually exists.

Why not just use one single loop with a variable setpoint calculated elsewhere?

That is exactly what a cascade is — the "elsewhere" calculation is the outer loop. Structuring it explicitly as two loops (rather than one loop with an ad hoc setpoint formula bolted on) is valuable because it lets each loop be tuned and diagnosed independently: PID gains for the fast inner loop don't need to account for the slow outer dynamics, and the outer reset logic can be as simple as a linear schedule without needing its own PID tuning at all.

Can a cascade have more than two nested loops?

Yes, though it is uncommon in typical HVAC/BAS applications. Process industries occasionally nest three loops (e.g., a temperature loop setting a flow loop's setpoint, which in turn sets a valve-position loop's setpoint) when the intermediate dynamics genuinely differ enough to benefit from separate tuning at each stage. In smart buildings, two-level cascades (a reset schedule feeding a PID loop) cover the overwhelming majority of real cases.

What goes wrong if the outer loop changes the inner setpoint too fast?

The inner loop is chasing a moving target rather than a stable one, which can produce oscillation or hunting that looks like a tuning problem in the inner PID loop but actually originates upstream. This is why reset schedules are usually rate-limited (e.g., no more than a few degrees of setpoint change per minute) — slowing the outer loop's effective response protects the inner loop's stability without touching its gains at all.

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Cascade Control vs Single-Loop Control

Why a hot-water reset schedule and a discharge-air PID loop working together isn't two separate control problems — it's one cascade, and the difference matters for tuning, troubleshooting, and stability.

A single-loop controller measures one process variable, compares it to one setpoint, and drives one final control element. That's the AHU discharge-air-temperature loop most engineers learn first: measure the air leaving the coil, compare to a fixed 55°F setpoint, modulate the valve. Cascade control looks similar at a glance — it also ends in a PID loop driving a valve — but it chains two (or more) loops together, where the output of an outer, slower loop becomes the setpoint of an inner, faster loop. The distinguishing feature isn't how many valves or sensors are on the drawing. It's whether one loop's setpoint is fixed by the engineer, or continuously recalculated by another loop above it.