That pesky tub flange. You have the tile backerboard installed and everything is waterproofed. Everything except that gap between the backerboard and the tub flange. How do you waterproof that?
What to do with the tub flange?
Caulk the gap
Probably the simplest way is to caulk the gap with 100% silicone or a urethane sealant. First you would waterproof with your favorite liquid waterproofing membrane of choice. Ideally you would leave 1/8 inch gap which would then have sealant applied to it. Either stopping the cement board above the tub flange or alternately shimming it out and dropping it down near the tub would both be good options. Shimming it out is probably only feasible if the tub has a really skinny flange like a lot of the steel tubs do.
Band the flange
Seal and (mesh) tape the seam
This is what I’ve been doing lately. I usually have some Wedi glue in my van and have been placing 2 inch alkaline resistant mesh tape over the seam and gluing it with wedi glue. Utilize whatever you have- Wedi glue, Kerdi fix, or a waterproof polyurethane sealant from a hardware store. Using a small putty knife spread the sealant smooth across the mesh tape. The mesh tape makes me feel a little better about bridging that seam.
The reason that I don’t recommend the mesh tape with step one (silicone) is that the silicone will act as a bond breaker to thinset if it’s smeared over the surface. With Wedi glue (or something similar) you can adhere tile to the surface of it. So with method #1 you want to keep the sealant as clean as possible and in the gap only- not on the surface.
There you have it. Three different ways to treat the tub seam. None according to instructions. All preferable to doing nothing.