QUESTION ABOUT VX ACE TILESETS

If you hover over the name of the part you want more information of in the Database a pop up will appear to tell you a bit about it. In this case it tells you that there's more information under the Resource Standards area in the help file.

This is a very handy feature that you should keep in mind for if something confuses you in the future. (It also works in most areas of the engine - just hover over the title of most aspects and info will appear.)

To break it down a bit, though, here's your answer:-
MODE:
The purpose of the tileset. This mainly impacts the special specifications of lower-layer tiles and the handling of battle backgrounds.
As a general rule, select Field Type for tilesets representing world maps, including oceans and landmasses, and Area Type for all others.
To use resources created for RPG Maker VX, select VX-Compatible Type.

specifications of lower-layer tiles
The A2 tiles in the tileset under the tile palette's A tab (rows three through six) are divided into base tiles (columns one through four from the left) and embellishment tiles (columns five through eight from the left). Embellishment tiles can be placed on top of base tiles.

However, with tilesets where Mode in the tileset dialog box is set to Field type, placing embellishment tiles over base tiles selected from columns two or four transforms them into column-one or three base tiles.


Specify Battle Background
When selected, this setting allows you to specify a combination of two graphics to display as a battle background when a battle occurs on this map.

When it is not selected, processing depends on the Mode of the tileset that was set for this map.

Mode is Field Type
The battle background is automatically determined by the tile where the player is standing when the battle occurs.

Mode is Area Type or VX-Compatible Type
The map and its effects will be used for the battle background.