I have been reading some text which indicates that to index an XML column, there must be a clustered index on the Primary Key of the table, and that the index on the XML column must be a "Primary Index".

I understand a primary key, and I understand that a primary key can be a clustered or not-clustered index. An index can also be defined on the primary DB file, but I have not heard of 'Primary Index' before, and I am unable to pull anything up in a search for the whole term in BOL. Is there such a thing?

Your help is greatly appriciated.