The lowest pressure in the pulmonary artery just before systole. This is often used to estimate the volume status of the left ventricle.

The peak pressure in the pulmonary artery during systole.

After systole the ventricle relaxes in diastole. The pulmonary artery now has greater pressure than the ventricle and blood tries to rush back in but the PA valves closes and with a bit of elastic rebound creates the notch.