There are two basic types of thread in bobbins, spun and filament. Spun bobbins are much more common because of the price. It’s easy to identify spun bobbins because they’re hairy, or rougher than top thread. The fibers are more loosely manufactured so they tend to come off and clog up the bobbin case.
Filament bobbins, on the other hand, are just like top thread. They’re a tightly-spun polyester that does not shed, which means less machine maintenance. And, many embroiderers claim they get less breakage with filament bobbins.
—Mark Stephenson, Colman and Company