The value of a variable (local or global variable, local or global parameter) may be referred to
using the construct
$name, where name is the variable name.
The variable is always bound at the textual place where the expression containing it appears;
for example a variable used within an
xsl:attribute-set must be in scope at the point where the
attribute-set is defined, not the point where it is used.
A variable may be declared to be of a particular type, for example it may be constrained to be an integer, or a sequence of strings, or an attribute node. In a schema-aware environment, this may also be a reference to a user-defined type in a schema. If there is no type declared for the variable, it may take a value of any data type, and in general it is not possible to determine its data type statically.
It is an error to refer to a variable that has not been declared.
Starting with XPath 2.0, variables (known as range variables) may be declared within
an XPath expression, not only using
xsl:variable elements in an XSLT stylesheet. The
expressions that declare variables are the
expressions. In XPath 3.0, variables can also be declared in a
expression, as well as in the signature of an inline function.