The larger the number, the finer the thread. 100 would be very fine. Size 50 is standand sewing thread. If it is super strong it must be polyester..... try burning a wad of it in an ashtray. If it burns, it's cotton. If it melts, it's poly or some other man-made fiber.
I like to sew my quilts with 100% cotton thread because I've heard that pure polyester is so strong that over time it will cut the cotton fibers in your quilt. When I've used cotton/poly thread I picked something that wasn't 100% poly and haven't had a problem as yet.
I'll be interested in hearing how this thread works for you.