The only place that needs to be caulked
is the floor tile to wall joint...unless you're looking for a
waterproof installation and then I'd go a totally different way

I'm trying not to go too far off topic here, but the backer board is
going over my entire bathroom floor in preparation for ceramic tile. I
also plan to put the baker board under the tub primarily because it's
the easiest way to keep the tub level with the rest of the bathroom.
(Tub is new install). I assume you mean waterproof as in what you'd do
with a shower floor or something like that, so no, there isn't a need
to water proof. I'll leave a slight gap between backer board and wall
1/8"-1/16". I hadn't considered the reason for the gap is for the
expansion of the wood wall studs. That was a good insight.

Oh, when the time comes shouldn't I also caulk where the tile meets the
tub?


Finally, in true amature form, I don't know the difference between
modified and non-modified thinset, but from what I've read I'm to use
non-modified thinset to bond the backerboard to the plywood. Another
source said to just use constructive adhesive. Any thoughts on that?

Modified thinset has latex additives. It allows for the materials to
flex more - a good thing. I use it exclusively.

I'll probably go with modified then. Although, if you google on
"modified" and "backer board" you might be surprised to find that
there's some debate on this. I don't have the knowledge or field
experience to sort it out.

I guess I'm ready to move forward from here. Thanks again. I really
appreciate people like you taking the time to help out newbies like me.