Hi Jackie,
the higher the thread weight, the finer the thread. I think there are some topics about this on this board. I just checked my Mettler thread and it just has No 40 on them, I'm not sure if that's the weight, but it seems finer than 40wt to me.
I would recommend 50wt Aurifil for applique generally although I have also used 40wt and it still looks good.
Glenda