Complex sentence: Top bars may be hidden under the ribbon so they are not visible, be a plain device from which the ribbon attaches or even decorative to complement the design on the medal; some top bars are elaborate and contain a whole design unto themselves.
Split into simpler sentences: Top bars may be hidden under the ribbon so they are not visible, be a plain device from which the ribbon attaches, or may even be decorative to complement the design on the medal. Some top bars are elaborate and contain a whole design unto themselves.