Superior Threads put out some education about Sew Fine.
"Filament polyester is a continuous fiber thread. Some hear the word filament and incorrectly assume it is monofilament. Monofilament, which looks like fishing line, is just one type of filament thread. It is a single (mono) strand thread. Other filament threads are multiple filaments, which consist of two or three strands twisted together. This is the largest category of filament polyester. Multi-filament strands are smooth and lint free but are not transparent. The advantage of a lint-free thread is a cleaner machine and less maintenance. The Bottom Line and So Fine! are examples of this filament polyester thread.
Today’s polyester threads are nearly indistinguishable from cotton thread, and the polyester’s modern formulation has no sharp edges…only smooth fibers. You can use modern polyester thread without the fear that it will eventually cut your fabric. In fact, polyester is an excellent choice when the quilt you are working on will receive lots of washing or use. The polyester thread holds up much better than any cotton thread under these conditions, increasing the longevity of your quilt.