Figaro - The XML Database for the .NET Framework

The Key Type identifies what sort of test the index supports. You can use one of three key types:

Key Type Description
Equality The performances of tests that look for nodes with a specific value.
Presence Improves the performance of tests that look for the existence of an node, regardless of its value.
Substring Improves the performance of tests that look for a node whose value contains a given substring. This key type is best used when your queries use the XQuery contains() substring function.