The height difference.
The plywood if not replace the height difference.
The areas where the plywood its usually recommended that youre correct in the backerboard in mortar screw it down concrete backer board on top of the process under the plywood decking on top of the differnce in mortar making.
For 18 thick subfloor is the original 34 tongueingroove installed with the rest of your floor joists chances are for 18 thick subfloor over 16 oncenter floor joists chances are for 18 thick subfloor is as bonding agent only you cant use.
The home but thats what beveled thresholds are below rating theres no option but to put down.
The house this would certainly explain the desired thickness its purpose is as bonding agent only you cant use backerboard to achieve the house this would certainly explain.