The map editor is in Foe mapping mode (also known as "Foemap mode").

F2 switch to picture mode
F3 switch to passability mode
F4 switch to door placement mode
F5 switch to NPC placement mode
F6 switch to foemap mode

Use the arrow keys to move around the map.

Use SHIFT+arrow keys to move around the map faster.

Foe mapping allows you to specify where random battles can happen. Whenever the hero walks on a tile with a foe map number, the player has a chance of encountering a random battle.

The number is the same as the Formation Set ID number from the Formation Set Editor.

Press < and > to change the current formation set number, or type in a number.

Press SPACE to mark the tile under the cursor so that random battles from the selected formation set can happen there. Only one formation set can be assigned to each tile.

To remove random battles from a tile, press DELETE (or change the formation set number to 0).

Press CTRL+F to fill the screen with the currently selected formation set number.

Press CAPSLOCK to change the selected formation set to the one under the cursor (eyedropper tool).

TAB toggles a blue rectangle that shows roughly what area of the map you are currently looking at.