Some of the menu elements appear only in this mode, so you have to switch between the two modes during your modding work.
The tools of the Editor / Layout Mode Edit
Aligning Regions Edit
Aligning terrains is necessary if you want to make larger maps. Aligned terrains touch each other but do not overlap. The best way to align regions properly is to switch the camera from "perspective" to "top view" and zoom in. Then move the terrain boxes by dragging the arrows. The terrains snap to grid every 1m, so that you can easily verify they are just touching if you zoom in far enough.
You need not align the terrain boxes vertically!
However, for aligned terrains to function properly in the game, you must make sure that the terrain elevation is equal all along the border. This ist best done in editing mode by using the plateau tool or the smooth tool.