frame horizontal up --> :topleft split
frame horizontal down --> :botright split
frame vertical left --> :topleft vsplit
frame vertical right --> :botright vsplit
window horizontal up --> :leftabove split
window horizontal down --> :rightbelow split
window vertical left --> :leftabove vsplit
window vertical right --> :rightbelow vsplit
NOTE: In VIM parlance, a "window" is "a viewport on a buffer," so splitting a "window" is splitting just the portion of the screen dedicated to the currently selected file contents. Splitting a "frame," then, is splitting the entire window (in the ordinary sense of the word window).