This module is devoted to the sub-graph nodes: the nodes, that that can have the other graph assigned.
Biome nodes allows mixing several graphs together, creating smooth transitions between them. This way you can make a green forest that is gradually changes with the desert at the south and with tundra at the north, and don't need to keep it all in one graph. Instead, you can use three self-sustained and independent graphs: forest, desert and tundra, and one parent graph that determines where each biome should be used.
Functions are even more advanced biome nodes: they can have their inlets and outlets. Imagine using a portal right into the biome sub-graph, and the portal out - and that is what the basic function is. This way you can re-use some common elements in all of your graph, bringing them to a separate graph. Tuning and enhancing it will automatically enhance all of the graphs where the function is used.
And more than that, functions can have internal parameters exposed. Consider it as as a parameter passed to the function. You can expose a field of the internal function graph, and set this field value from the parent biome. And the most exciting thing that the exposed field values are independent from the function graph - you can use two or more same functions with exposed different values.