11.1. XSLT Function Groups
XSLT function groups follow the same conventions as XPath core function groups. There are four XPath function groups aligned with the four objects they produce: node-set, string, Boolean, and number. However, there are no XSLT functions in the number function group, so there is technically no XSLT number function group.
The XSLT functions that return node-sets are: document(), which works with material from external documents; key(), which provides an index-like addressing capability; and current(), which provides a way to explicitly address the current node in a given context. These three functions are in the node-set XSLT function group.
The XSLT functions that return strings are: system-property(), which is used ...