Poly behind if you won't be applying a waterproof coating to the surface of the CBU. If you'll be waterproofing the CBU, not poly behind. If you do both, there's concerns about traping moisture between the layers. One or the other it what most will tell you.
Redguard is a popular waterproof coating. Schluter Kerdi is becoming very popular. It's a bonded waterproof sheet membrane. I use Prospec B-6000 but I don't think you can get this retail. MAPEI's Mapelastic HPG is popular as well.
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