XSLT Tutorial

In XSLT stylesheet 1 strings are arguments of boolean() function. A string is true if and only if its length is non-zero. In XSLT stylesheet 2 is text transformed into numbers and then subjected to boolean() function. XSLT stylesheet 3 compares "0" as a string and as a number. XSLT stylesheet 4 uses node-sets as arguments for boolean() function.

http://www.zvon.org/xxl/XSLTutorial/Books/Output/example40_ch11.html