From @topisani on April 6, 2016 7:25

These will be mainly used for valcanoes for now, but the point is to have add something that is placed like a feature, with access to the distance from the center and such, but instead of directly placing blocks, it will be able to act like a biome, manipulating the terrain and surfacing.

Copied from original issue: Team-RTG/1.9-Realistic-Terrain-Generation#63

From @topisani on April 24, 2016 11:9

this might be the solution to quite a few things, lakes and posibly rivers should be terrainFeatures as well.

These are theoretically implemented, but still need some work