Servo - Fixed Wing Attitude Controller

Hi everyone,

I am trying to build a transfer function model of this FW Attitude Controller diagram.

The problem is that I have not found how the mixer block works. Does anybody know where I could find how the command u is calculated?
For example, in this code, PX4-Autopilot/ecl_pitch_controller.cpp at master · PX4/PX4-Autopilot · GitHub the output parameter seems to be _last_output, which is constrained between -1 and 1, but I imagine that the command u is the value of width pulse (expressed in ms or us), so how is this calculated?

Thanks in advance, regards.