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This paper provides a good overview of the Kinect measurement model:

Accuracy and Resolution of Kinect Depth Data for Indoor Mapping Applications

In the paper, the authors refer to a parameter Z_0, or the distance to the reference plane. I assume this is what you're asking about. They calibrate for the depth readings, so if you go through the equations, you can find the value of Z_0.

Now in the model the authors describe, they only use a single Z_0. However, it is entirely possible that the Kinect uses multiple and switches dynamically, but that could be hidden from the user.