To expand further on this subject...

The X is pretty standard for the area to be cut out. So that is a good
start. What about the best set of symbols for the six sides of a cube?
Should the symbol incorporate information as to what side attaches to
each edge? Is "right reading" enough to define its position and what
side attaches to each edge? What other information should it carry.
What have I forgotten?

I hope you all don't think I am being anal retentive about this. Being
able to come up with a marking without thinking should make work go
quicker. It also would keep you from second guessing yourself later in
the project (was B bottom or back). Both of these problems have
happened to me. I have refrained from coming up with my own markings
because I have been looking for an existing standard that has been well
thought out. The cabinet maker's triangle is an excellent example. It
was obviously well thought out and almost impossible to screw up. It
also can be used for an infinite number of panels. See what I mean?

There is a method of marking the 4 posts of a cabinet to understand
which post is what and its orientation. I use it all the time and it
is not ambiguous. Makes my work go quicker and with more confidence.