Functions are in the
http://www.w3.org/2001/12/xquery-functions
namespace which is customarily mapped to the xf prefix
The function namespace name and prefix is understood in XSLT, without being explicitly stated.
Operators are in the http://www.w3.org/2001/12/xquery-operators namespace
XPath implementations such as XQuery and XSLT map the operators to symbols like * and +