Pigtail Bobbin

looking online I found this explanation that might help from Bernina:
The Embroidery Bobbin Case is often referred to as a pigtail bobbin case because of its double looped tension wire. It is also called a gold latch bobbin case because it has a gold latch.

The double looped tension wire on the Embroidery Bobbin Case changes the angle at which the bobbin thread is aligned when the stitch is formed. This gives a tighter tension to the bobbin thread and enhances the smooth, professional look of your embroidery design. It can also be used when sewing with decorative stitches, sewing buttonholes or satin stitching

I had never heard of it either.. but it does sound interesting.

Our bobbins cases have pigtails on them. It's just a little curl of wire, which does look like a pig's tail that helps guide the thread. I thought those were standard on bobbin cases. Guess maybe they're not.