Tile backer board and bath

redoing a bathroom, we currently have a bath with a wall on each end and wall on the back(obviously)

I've started to strip one wall of tiles(not back to bath yet) it's plasterboard on dot and bad which ive taken back to block work. wall behind bath will be dot and dab and the other end will be stuff work.

when putting the room back together, fitting the bath with tile backer boards does the board need to go back behind the bath(bit flanged bath) or rest on the rim of the bath?

also it will be bath with shower on one end.