Recessed Tub Flange
Stud notches and sheathing strips plane the wall surfaces out with the inside edge of the tub flange so the backer board clears.
The American Standard 30 in. x 60 in. tub in the main bath has a perimeter flange that the backer board and / or finish surrounding walls (tile, plastic) are supposed to lap over. The challenge is dealing with the flange thickness so the walls are in uniform planes without a bump along the bottom.
To get the stud surfact to plane out with the flange I sized the opening 3/8 in. narrower and 3/8 shallower than the standard rough space and notch out the studs on one end and at the rear of the tub. Notching is easy if you plan ahead. Before we erected the walls after nailing on the top and bottom plates I snap chalk lines a the top of the tub flange height and below the tub platform level. A quick pass with a circular saw along the lines and whack of a hammer clears out the recesses. Even if you didn’t make the notches ahead of time, it can be done once the walls are up.
After sliding the tub in place we padded the wall at the opposite end of the tub with leftover strips of sheathing.