If not mistaken, the higher the number the thinner the thread. You'd think it would be the other way around but I guess they like to keep us on our toes. You should be able to use it for both piecing and quilting. Heck I use whatever I have in the color I have it. I do stop myself when I find the thread is super old. My mother dumped me with a bunch of thread she had that was actually the 1st wife's thread from probably back in the 50's as some were on wood spools. Those I tossed but the rest of it I'm using for piecing just to get rid of it. If it snaps easy it gets tossed too. I have 10 large spools of medium gray thread to use up so I'm hoping for a darker fabric quilt to work on soon to use it up. Must have been for uniforms back whenever.