a pieced top and/or bottom. Not always sandwiched or quilted. Summer quilt skins are pieced sheets when you want the look of a quilt but not the bulk. Referencing to the outside of a quilt.
i call these summer quilts, i didn't know this had another name, i made a quilt with applique on a sheet, then put another sheet on the back and stitched them together, it works beautifully as a sumer quilt