When the backside of the helm area is enclosed with a wall to the cockpit. Tuna.com has a winter-back. A working lobster boat with NO wall separating the helm from the cockpit has NO winter-back. Hope that made sense
I have a canvas winterback on my boat. It consists of 3 pieces, center which goes on the two poles supporting my overhead and velcros onto the overhead, port side which zips onto the center piece and velcros onto the overhead and side of the cabin, the starboard side zips onto the center and velcros onto the overhead and zips onto the side panel, the side panel snaps into place top and bottom, velcro on the forward edge near the hauler.