This is a bobbin holder for the Bernina 500 & 700 series. This will attach to the front of you sewing machine with double sided type. I use the Clear thick double sided type. https://a.co/d/gbFiKHq Here is a link to it on Amazon. This is one my husband designed. The bobbin needs to go on with the black side out.

This just keeps a couple of bobbin handy while you are sewing.