Wall.png(A3, A4 even line)
Ground.png(A1,A2 and A4 odd line)
Table.png(A1,A2 + table option)
It is used for auto tile editing. Place this template on the background layer of the auto tile.
The same symbol as ● is a guideline with the same pixel.
And, check this page for understanding auto tile.
http://blog.rpgmakerweb.com/tutorials/tutorial-making-an-autotile/