So, I have been doing a lot of research and could not find a solid answer. There seems to be different opinions.

For my half-bathroom renovation, I am starting fresh. For flooring, for the first layer I am using 23/32 Advantech T&G, which is glued to the joists and then screwed down with 1-3/4" screws. Now, I want to install a second layer, 3/4" plywood, to meet the minimum 1-1/8" thickness for the large format porceline tiles I picked out.

Now, what I came across is that some people claim this second layer to be "underlayment" and some argues that it is still subfloor (second layer of subfloor) -- and with that, they offer two different installation methods:

Method 1: If it's underlayment, then you don't glue the second layer to the first layer, and should use a shorter screw (1-1/4") to fasten the 3/4" ply to the first layer without fastening to the joists. And maybe put some kind of felt paper in between the boards (optional).

Method 2: if it's the second layer of the subfloor, then you glue in between the boards, and use longer screws so the second layer, 3/4" ply, can also be fastened to the joists.

I understand the installation method, but I can't figure out what I am doing would be considered underlayment or second layer of subfloor?

I will also laydown 1/4" uncoupling membrane (Schluter Ditra) on top of the ply before installing the tile.