Current sensing application
The circuit keeps constant current because U7, U6, and transistor Q1 form a control loop with deep negative feedback. It is a standard linear regulator. The current level is defined by DAC output, Iout = DAC_OUT/500. If any deviation in current occur, the amplifier U6 will act to compensate/negate this deviation. All this is actually written on the schematics, with details on sensitivities and conversion ratios. The Zener is likely for protection against some spikes in the "load".