An orifice plate is a plat eused to measure flow. Pressure sensors are placed upstream and downstream of the the plate and hepressure differential is used with Bernoullis Equation t compute flow (including some experimental factors).
A retriction plate is similar to an orifice plate but is much thicker. It is used to drop pressure upstream of a system. The convergence, throttlnig through thick section and divergece works the fluid and reduces total energy (transformed in firction, heat etc). This leads to an overall pressure reduction.