This tool allows you to align the top faces of plates. It can be useful in situations such as a concrete slab that consists of plates with different thicknesses where there are drop panels or beam thickenings. By default the slab plates will all be aligned along their centerlines and so the top of the slab will not be flat. The align plate faces tool will apply offsets as necessary to the plate elements so that they all line up along the top.
After selecting the plates to be aligned, right-click and select "Plate Tools" => "Align Plate Faces" from the popup menu that appears. You should then click a plate that the selected plates are to be aligned with."