Darkoshi

If a variable has a select attribute, then its value will always be one of the four basic XPath data-types (string, number, boolean, node-set).

If a variable has empty content and does not have a select attribute, then the value of the variable is an empty string. Thus

	<xsl:variable name="x"></xsl:variable> 

is equivalent to

	<xsl:variable name="x" select="''" /> 

A variable with non-empty content ALWAYS returns a value of type Result Tree Fragment (RTF).

A RTF is "treated equivalently to a node-set that contains just a single root node."
I don't understand what is meant by "treated equivalently", considering that you can't perform XPath expressions on a RTF.

However, for a variable of type RTF, this kind of test will always evaluate to true:

	<xsl:if test="$RTF"> 

as a single root node is a non-empty nodeset, and non-empty nodesets get converted to a boolean value of True. Therefore when $RTF is converted to a boolean, it evaluates to true.
That is the case even when the variable is defined like this:

	<xsl:variable name="RTF">
		<xsl:if test="something that evaluates to false">
			...
		</xsl:if>
	</xsl:variable>

For this variable, both of the following evaluate to true! (even though an empty string would normally be equivalent to false)

	<xsl:if test="$RTF">
	<xsl:if test="$RTF = ''">

<xsl:value-of> always returns the *string-value* of the XPath expression, boolean or otherwise.
(see http://cygwin.com/ml/xsl-list/2000-10/msg00198.html)

<xsl:variable name="RTF">
	<xsl:value-of select="'abc'" />
</xsl:variable>

In this case, $RTF is *not* a string - it is still a result tree fragment.
But when a RTF is compared to a string, apparently the RTF is converted to a string.
So this condition evaluates to true:

	<xsl:if test="$RTF = 'abc'"> 

and this evaluates to false:

	<xsl:if test="$RTF = 'abcd'"> 

...
Distinguishing between an empty node-set and an empty string...

If your XSL template has an optional parameter which may or may not be passed,
and which may be an empty or non-empty node-set when it is passed,
then it is hard to distinguish between the parameter not having been passed, versus an empty node-set having been passed, as in both cases, comparing the parameter to '' returns true.

One way around this problem is to give the parameter a default non-empty string value which the passed node-set should never contain, and then to check if the parameter's value is other than that default value.
If the parameter is passed and non-empty, then it won't equal the default value.
If the parameter is passed and is empty, then it also won't equal the default value.

<xsl:param name="inputNode" select="'_not_passed_'" />

<xsl:if test="not($inputNode = '_not_passed_')">
	<!-- if true, this means the parameter *was* passed. -->