What is matrix_basis? I'm trying to use Blender as my GUI content creator, and I'm writing plugins for it. 

I have a brief look on the API document and noticed that Objects have three matrices: world, local and basis. It is pretty easy to understand world and local matrix, and they work as I expected. However, I don't get the concept of basis matrix, and it can be disturbed by Alt-O hotkey.

Although it seems local and world matrix is enough for me, I am still curious about the meaning of the basis matrix. So what does basis matrix stands for?
From the documentation it states:


  Matrix access to location, rotation and scale (including deltas),
  before constraints and parenting are applied


The key is that it is the matrix before constraints are applied, as some constraints can alter the local matrix.

API docs - bpy.types.Object