The Avid X0 Trail brakes have increased stopping power compared to the regular X0s due to the new caliper with a two pistons instead of one. But, like the standard X0s, the trail brakes have a top loading pad system that makes removal and installation a snap.
A steel backing plate comes in two options: Organic or Metallic. Organic pads are softer and will provide quieter braking and increased modulation. Metallic pads will provide a longer life and improved heat dissipation. And because of this, they're recommended for heavy duty applications. It is important to note, though, that they'll be louder.
The Avid X0 Trail Pads are sold as one pair of left and right to fit one caliper and include the retraction spring and retaining screw. Again, you have the option of either Organic or Sintered.