Tiling backer and tanking?

I'm moving my bathroom upstairs, to where a bathroom should be. Its an old house. It has wooden floorboards, and i will be taking off all the old plaster from the walls. The wall that will be the back of the shower/bath is a stud wall. I intend to tile most of it so was planning to do the floors with 6mm tile backer boards, and the walls with 12mm tile backer boards which I can screw direct into the wood/brick.

My question comes with tanking, it's new to me and I've seen conflicting information. Some people say I don't need to tank when using tile backer boards (other than seal the joints obvs). Others say i must tank around the bath and shower, by putting a membrane over the backers before the tiles. I've also read that waterproof membranes should be put down UNDER the tile backer boards. But that the bath should sit direct on the wooden subfloor and shouldn't have waterproofing underneath it.

What's the reality here?