2 is a logic valve, it basically makes a small LS closed centre system inside this valve block for the autosteer. This valve will keep the P line a few bar higher than the internal LS line (in other words a few bar higher than what is needed to operate the wheels at the current time)
1 is a relief valve used to limit the maximum pressure for the steering, when this valve is tripped the LS line can’t go any higher and the valve 2 will let the oil out the T port
The pressure sensor is connected to the valve block T line, this line is connected to the P port of the orbital and because it is open centre the oil free flows to tank. But when you turn the steering wheel the pressure in this line rises as oil is needed to turn the wheels, because the orbital steering lines are blocked this pressure rises quickly and the sensor trips to tell autosteer to stop steering