Blends multiple biome layers the same way texture layers are blended together with the Textures Output. It creates a smooth transitions between the biomes.
Biomes Set has a normalizer built-in. It blends layers and normalizes them so that their sum is always 1. This way the layers order matters - each upper layer will overlap the bottom ones. Consider layer masks as Photoshop layers masks.
The Background layer does not require an input since it is regarded as completely filled (a constant of 1).
Note that for the best blending effect biomes should have the relief matching as close as possible - if some biome has a mountain or a plane in some area then all of the other biomes should have the mountain or a plane here. This could be achieved by the main Noise generator the same Seed, Size and Offset values for all the biomes. See the included demo biomes Badlands and Snow Plains as an example.
Use Cases
The typical use scenario assumes assumes assigning large-scale maps as inputs. They could be either multi-tile gradients (Simple Form), or noise with the Size value of 1000 or more, or any other map with a pattern that spans across multiple tiles.
Mixing two biomes (green and yellow) with Noise, and adding snow biome from north and south.
Properties
Biomes use the standard Layers interface. Each layer contains the graph slot, where the biome graph should be assigned. The layers could be moved by clicking and dragging the biome icon 
.
The graph could be opened as a biome by clicking the Open button 
. In contrast to opening the graph by double-clicking it in Project window, accessing it with the Open button will open it as a biome. You can return to the parent biome with the Folder Up 
button in the toolbar, and access other biomes with the Folder Tree 
(also in a toolbar).