If the plant you select is a process model (idproc object), PID Tuner uses its structure.
If the plant is any other model type, PID Tuner uses the state-space model structure.
Switching Between Model Structures
When you switch from one model structure to another, the software preserves the model
characteristics (pole/zero locations, gain, delay) as much as possible. For example, when
you switch from a one-pole model to a two-pole model, the existing values of T 1 , Tz, τ and
K are retained, T 2 is initialized to a default (or previously assigned, if any) value.
Choosing Identified Plant Structure