A plate is defined by three things: flatness, parallelism and uniform thickness. Get those right and everything assembled on it behaves; get them wrong and the error propagates through the whole stack.
Base plates, cover plates, separator plates, mounting plates and vacuum or fluid distribution plates, with bores, threads, pockets and sealing grooves as required.
Flatness across the working face, parallelism between faces and thickness consistency. These are measured on a Zeiss CMM rather than inferred.
Aluminium, stainless steel, titanium and engineering plastics, depending on whether stiffness, corrosion resistance, weight or insulation dominates.