This tag is for the property of two different things being coincident or making them so. For example a land cover map and a land use map may have different boundaries for the same area. One could be aligned to the other, or both aligned to a parcel map so that all shared common, coincident boundaries with no overlap or gaps.
Note this term is also used to refer to a specific path or route, ie a road alignment. However for the context of this tag it should only be used for coincidence.