CAD.Reference:4.1 - The recurring whole problem

A layered control architecture divides decisions while controlling one connected behavior. The division is useful when it lets different calculations use appropriate information, horizons and execution resources. It becomes harmful when each calculation relies on a behavior that the others do not supply.

The difficult question is therefore not just which operation follows another. It is whether the chosen decomposition, models, command meanings, error bounds, feasible commands and execution arrangement can all be true of the same closed loop. A later result can change an earlier design premise. The four Methods are separately usable because these questions arise independently; their results must still agree wherever one receives another.

Extended reminder. A useful control split preserves the whole task by making its lost interactions explicit. The exchanged command and its timing determine the error that the lower behavior must contain; the containment consumes state clearance and actuator authority, so it limits what the upper decision may request. Those limits reshape the useful model, horizon and decomposition, while the resources needed to compute and execute them change timing again. Compare and realize this connected choice under one demand, and return a changed condition to the premise it invalidates.

This reminder names a coupled argument. It does not prescribe four approval stages, prove a bound, or replace the constructions below.