implementation details - How to interpret the FullForm of a SparseArray?

SparseArrays are atomic objects, but they do have a FullForm which reveals information about them.

What is the meaning of the various elements in the full form of a SparseArray? Did the structure change between Mathematica versions, and is it documented anywhere?

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Update: In recent versions of Mathematica the LibraryLink documentation has a useful description of the internal SparseArray structure.

Answer

I have described the details here and here. The second post describes the version number of the sparse array implementation, which is still at version 1. So no big changes since it's introduction and V9.

If you like to read about sparse arrays I can recommend this from Tim Davis.