The specification for the XPath 2.0 functions is in the document named XQuery 1.0 and XPath 2.0 Functions and Operators, which you can find at http://www.w3.org/TR/xquery-operators.
The namespace for functions is tied to the fn prefix. Its URL is http://www.w3.org/2003/05/xpath-functions , and you can call these functions directly (although when you use Saxon, you omit the fn: prefix). The other prefix defined in the XQuery 1.0 and XPath 2.0 Functions and Operators document is the op prefix, which identifies the operator functions. These functions are not directly callable; these functions are designed to indicate how the XPath 2.0 operators work with various data types.
The XPath 2.0 functions fn:node- name , fn:string , fn:data , fn: base-uri , and fn: document-uri are generic functions that you use with nodes.
The fn:error function gives you some control over the processing of XSLT stylesheetscalling this function lets you display an error message and halt the XSLT processor.
The context functions return information about the current context. These functions are fn:position , fn:last , fn:current-dateTime , fn: current-date , fn:current-time , fn: default-collation , and fn: implicit-timezone .
The numeric functions in this chapter. These functions are fn:floor , fn:ceiling , fn:round , and fn: round-half-to-even .