V1 pin 23 PA0_T51, what does it do?

In the schematic of the DSO QUAD there is a signal “V1” coming from the pin 23 of U8 STM32F103. It is used to add an offset to the signal, by means of U17, before reaching the input of the ADC. Can someone say me if it is a static signal (fixed 0V or 2.8V) or a PWM signal (used to add a variable mean signal after the RC filter)? Can someone explain me where it is handled in the application software? I looked at the source code of the Gabonator and Wildcat applications but I coudn’t find any reference to this signal, but I’m not a good expert of software.

I’m asking about this because I noticed a great variation of performances of the DSO QUAD when is powerd by the battery or by the USB. The input stage scheme looks to be well designed, with a good rejection to the power supply variations. I expect that most of the power supply dependancy is related to the variation of the V1 signal. So I’m tring to understand how it is used to try to improove the DSO QUAD performances.