This tag determines if the variable is defined at the current moment in time. IISDEF will report false on environment variables since these are not defined until requested.
Usage	This can be used in combination with <iIF> tag to determine if a variable is defined.
Resolution	Resolves to true or false
Comments	varname is the name of an environment variable. You do not need to put ':' in front of the variable name in the VAR directive of the tag.