This plate is the back-up plate to all the of all ejector pinheads. It has stop pins mounted in it on the opposite side to keep the plate flat in the mold base. It has the knock out taps for the ejector rods, which requires it to be thicker than the retainer plate. It can also have water in it to keep the plate temperature stabilized with the rest of the mold base. If the mold base runs hotter than the ejector plate in a big mold the thermal expansion will cause the clearance to be too tight and the ejector pins will have premature wear and breakage.