The phrase Orifice Plate is actually a device that measures either volume or mass flow rate. An Orifice Plate is a thin plate like device that has a hole in the middle.

Bleed or Weep Holes are located at the elevational top or bottom of the Orifice Plate. The Weep Hole is tangent to a circle equal to 98% of the line I.D. DRAIN HOLES are located at the bottom of the Orifice Plate to prevent liquids in a gas or condensate in a steam flow from damming up behind the Orifice Plate. VENT HOLES are located at the top of the Orifice Plate to allow gases in a liquid flow to vent pass the Orifice Plate.

Orifice size corrected to reference temperature.

generally vena contracta is formed at distance of d/2 from orifice where d is diameter of orifice.

Because it relies on the back pressure through a known orifice which its differential pressure across the plate relates to flow.