Cement backboard touch shower pan?

Seems like there's two ideas about the cement backerboard touching the concrete shower pan. One idea is that they should not touch to prevent water from wicking up the walls from the cement pan. The gap between the floor and the wall is small, say 1/8", so it is filled with silicone caulk.

The other idea is to have the final layer of pan mud hold the lower portions of backer board in place. This means that the backer board does not need fasteners near the bottom. The fewer the fasteners, especially near the bottom, the less likelihood of a leak or failure.

Why is one way better than another?

 

Actually it's both. The cbu should be above the pan liner and if desired,sealed with a bead of sealant. It should also not be screwed through the liner below a point 3" above the curb height. The mud holds it in place at the bottom. With a preslope under the liner, the amount of water that would wick into the cement board is minimal and it wil dry out fast. Without a preslope under the liner, then you will have the CBU constantly sitting in a puddle of water. The cbu is kept up off the liner to prevent the stresses of the cement board from causing the liner to stretch and tear. Since the side of the cement board is sitting in the mud bed at the bottom, sealing it is sort of redunant as water will still wick intothe side of the cment board anyway.