sql server 2008 - How is a spatial index represented -

I have read the documentation about spatial indices on MSDN, but I do not understand two things,

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What are the keys to the index?

How are the sets of protected cells shown in B-trees and how they are.

The keys of the index should be polygon in that location (rectangular in this case) which is indexed Is being done (or equivalent, that is, they may use grid index), in this case they appear on regular, hierarchical grids.

As stated, the 2D index is used to map to 1dB-tree. This is to provide good spatial location between the nearest index.