The regular 3 different sizes of machine embroidery bobbins in the market: Style L, M, and A (also know as Class 15).
These do not stand for large and medium.
The L & Class 15 embroidery bobbins are the smaller bobbins, they are about the size of a nickel.
L fits most commercial embroidery machines and many home machines;
A or Class 15 fits many home machines only;
Style M machine embroidery bobbins are about the size of a quarter. They are also sometimes called jumbo bobbins. Your embroidery machine must have been manufactured to take the M bobbin, otherwise, they will not fit.