This bobbin case is a lower tension case used for sewing with thicker threads, such as cordonnet threads that cannot fit through the eye of a needle. 

Suitable for the 710, 750QE, 780

Also fits with:

New 4 Series: 480, 475QE, 435
New 5 Series:500, 535, 540, 570QE, 590
New 7 Series: 700, 720, 740, 770QE, 790, 790PLUS