Differential pressure based flow cytometers currently dominate the market. These systems have two pressure regulators. The first is at a constant pressure that sets how fast the fluids runs at. The second is regulated by the investigator (like as shown on this LSR-II control panel).
As the sample pressure goes from low, to medium, to high, the pressure on the sample increases. This results in the volume of the sample increasing (from ~15 ml/min to ~60 ml/min).
The difference between the sample pressure and the sheath pressure is the differential pressure. This controls the width of the core stream and the total number of cells passing the laser intercept.