Hi megamicro,
this is a two stage power supply: the first is step-up switching, the second is linear.
The linear regulator can provide accuracy and fast response. The switching source, acting as a preregulator, gives the necessary voltage to the linear regulator for working fine without bad efficiency.
You can control the duty cycle of the PWM signal injected at GVP taking the sample of output voltage of the first stage at A.
The linear regulator must be controlled taking the output voltage (VPP), comparing against a reference and controlling VPPE.
Regards
Z