The optional select attribute is an expression that defines the parameter. If the select attribute is present, then the xsl:param element cannot contain any content and is self-closing. If an expression is given, the data type must be Boolean, node-set, number or string. If it is a (literal) string, the string must be enclosed within opening and closing quotes and in turn, that default value must be enclosed again in opening and closing quotes.
<xsl:param name="car" select=" ' Ford ' "
/>
<xsl:param name="car" select=' " Ford " '
/>
If the name attribute is assigned, a select attribute is not assigned, and there is no content, then the named parameter is set to be the empty string for the default value.