I think it has to do with if the Caliper is fixed, and has pistons on both sides of the rotor, or if the caliper has pistons only on one side, and "floats" on pins back and forth.....
Course, I could be totally wrong. The nomenclature can vary by manufacturer, poster, sobrity, your results may vary, past performance may not indicate future results, with approved credit, plus Tax, Title, and License fees.......
Rob