First thing I do when I start screen is `C-a S` to split. The second thing I need to do is to open shell in the new window - `C-a c`.
I use screen only occasionally and each time, I can't remember this `c`. (I actually don't remember even the `C-a S`, I have it in my .screenrc :-).) So I do `C-a ?` and stare like fifteen minutes saying to myself "come on, blind man, such a basic thing must be there, you're just not seeing it", but that never helps because it's *not there*. Finally I somehow magically remember but to be honest, it's a PITA.
I suggest adding this `C-a c` to `C-a ?` help screen, at least for old blind & forgetful people like me.