The drawing of tiles is controlled here.
The Mode allows the drawing mode to be selected. The different modes
are explained here.
Seq. Draw activates the sequential drawing mode, which is explained in
more detail here.
Zoom allows you to zoom into the map for easier editing. Note that zoom
level zero draws the tiles half sized, so that an overview of a map can be seen
a bit more easily.
Tile selects the tile to be used for drawing, and the start tile in the
sequential drawing mode.
Flip Horiz (only when editing non-rotational maps) means that the tiles
will be drawn horizontally flipped. Flip Vert does the same, but
vertically.