In general I understand PID controllers. But I just used P I D parameters to tune the controller. Here there are more tables. I need to read more the FR.
The reason IMX is so important in ME7 is that in the steady state, P result is near zero and D is scheduled out with B_lddy. This means that I dominates.
On a WOT pull, I is maxed out due to integrator windup (accumulated error) so it is simply following IMX.
Also in the steady state, you would expect the output of the PID to be flat for a given flat boost set point. However, DC isn't linear with output boost, so the output of the PID is post processed by DRL.
Again, if you truly understand PIDs, you should already know the critical points here
1) integrator windup effects when there is long term error
2) linearizing the control output to compensate for varying process output gain.