Here mean the contents of biome.xml.
biome.xml, along with blocks, in addition has the ability to use prefabs as decorations (burned remains of houses, rock formations, piles of coal or nitrate, and etc.).
ie, some prefabs are used as decor, and controlled via biome.xml.
In fact, this used only the content of the prefabs - the blocks. In this case - the placeholders, sleepers or static spawn - don't work.
With the class "SpawnEntity" - becomes possible to spawn entity into prefabs which is part of the scenery biomes. (not RWG!)
Such previously was not available! Now it is possible to control spawn through decorations of biomes!