It is a great idea to have a bobbin case just for your cardboard pre-wounds, one just for the bobbins you wind yourself, and one just for your magnetic core bobbins. That way you just grab the right case for the kind of bobbin you are running and go. It reduces the amount of fiddling it takes resetting the bobbin tension with each type of bobbin change.
These APQS approved cases are specifically for APQS machines although they may also fit other longarm machine brands. The bobbin cases have a check spring in the bottom that can be removed. The “M” case also has a thread pigtail guide that some like to use and others do not. The “L” case does not have a pigtail guide.