Diffused light disperses light evenly across a subject and softens shadows. In general, it produces a more flattering image.
The Kelvin Epos Medium Diffuser D90 offers a slightly thicker diffusion, but with minimum light loss, less than 0.2 of a stop, and extends the beam angle slightly which makes it a better match for larger softboxes and also for less precise optics.

Key Features:
Medium diffusion for Epos
Widens beam angle
Allows for better color mixing with less precise optics
Magnetic mounting