Where to use the air speed sensor in the Ardupilot code

Hello…
I just bought an air speed sensor and wanted to add it to my plane and wanted to know where the sensor is used in the Ardupilot code…
I have been working on the code for a while, but I could not understand where exactly we use the air speed sensor readings, for example: are they assigned in auto mode or FBWA, etc.

Thanks in advance