The right hand side of the triangle represents mirrorless. The EOS-M is totally within this section as it is totally mirrorless. The 650D is half on the mirrorless section as it has composite auto-focus for video and liveview mode.
"What goes here?" - a new camera.
"What does this little arrow mean?" - the new camera will be either fully mirrorless or at least have composite phase/contrast auto-focus like the 650D does.
As opposed to partially mirrorless?