We've updated our case series names. Edge (formerly Ultra-Slim), Cairn (formerly ProGuard), and Summa (formerly UltraGuard) now reflect the experience, not just the specs. Edge: slim, light, and sharp along the climb. Cairn: firm and dependable, like stone markers that guide the way. Summa: the peak, where protection and hand-feel come together.

A. Essentially, 600D aramid fiber is a thinner thread, whereas 1500D aramid fiber is a thicker thread. The 600D version will feel more delicate to the touch and the case is slightly thinner.
You can read more about the difference here.