Minecraft wiki entry for Development_versions, associated text: Added <structure>.dat files in data folder for storing structure data.
Contains .dat called Village.dat (villages), (Fortress.dat (nether fortresses), Temple.dat (desert temples, jungle temples, and witch huts), Stronghold.dat (strongholds), Mineshaft.dat (mineshafts), and others.
NBT structure consists of the individual parts of each structure, labeled "children," the chunk it originates in, a tag called BB, and an ID for what type of structure it is.
The children tags have information on each part of the structure and the type of part it is