one question regarding AttributeSelector XACML Element

Dear all,

I have a confusion regarding the use of AttributeSelector's attribute RequestContextPath . As the XACML specificaiton says that 
An Xpath expression whose context node isthe XACML-context:Request  element .

Now if 

  1.. I want to read the data which is in XML format , but not from inside the request as specified under Request-Context but from a file which is located outside. is this possible with current XACML , if yes , then whether this RequestContextPath is not misleading ??

Regards
Muhammad.