I purchased the sofa cover and seat cover set.  The seat cover went over the top.  When the kids sat on it, sometimes it would ride up and reveal the cushion underneath.  I purchased the cushion cover so I had 2 to completely wrap the cushion.  It solved the problem and is holding in place.  I do think it’s slightly smaller than the one that came in the set.  For me it didn’t matter because I put that on the bottom and the other one on top.