XML indexes in SQL Server

What are types of XML indexes in SQL Server?
Microsoft SQL Server supports different types of XML indexes. An XML index is different than a relational index. There are basically TWO types of XML Indexes viz., Primary XML Indexes and Secondary XML indexes. The Primary XML index is a clustered index on an internal table known as the node table that users cannot use directly from their T-SQL statements. To enhance search performance, we create secondary XML indexes. These create secondary links (RID) at leaf level for existing clustered index based KEY pages. A primary XML index should be created prior to creating the Secondary XML Indexes.