Bobbin thread and prewond bobbins are usually polyester like serger thread, tex 25, but are lots stronger than cotton.Some prewond bobbins are also cotton, but are usually size 60. so is Coats & Clark dual duty cotton that we have used for years on both top and bottom for clothing, quilts, mending, etc.
The only thing we have to watch when using poly is not to iron the thread as it will melt, so iron, press on the right side only.
Bottom weight is made for bobbins for embroidery and prewond bobbins. I would not use it for top quilting.
Phyllis 81 yrs in Oregon