The Biome Painter is similar to the Painter tool, except it paints biomes instead of blocks. Biomes in Minecraft are represented by a 4x4x4 cell of blocks stopping at world height. Though biomes are stored in cells, they are altered in the rendering to make their shape irregular.
You can select any biome from the dropdown, including modded biomes sent to the client.
The Fill Vertically option fills the entire span of the world height when enabled, allowing users to quickly paint the entire column.
The Visualize Biome toggle is on by default and shows the 'real' position of biomes in the 4x4x4 grid.