The xsl:param element is used to define a formal parameter to a template, or to the stylesheet.

As a template parameter, it must be used as an immediate child of the xsl:template element. As a stylesheet parameter, it must be used as an immediate child of the xsl:stylesheet element.

There is a mandatory attribute, name, to define the name of the parameter. The default value of the parameter may be defined either by a select attribute, or by the contents of the xsl:param element, in the same way as for xsl:variable. The default value is ignored if an actual parameter is supplied with the same name.

There is an optional attribute, as, to define the type of the parameter. The actual supplied parameter will be converted to this type if required. If the parameter is omitted, the default value must conform to the type. Note that if no default is specified, the default is a zero-length string, which may conflict with the required type.

The type-information attribute is removed at Saxon 7.5

The required attribute can take the values "yes" or "no". This isn't allowed for function parameters, which are always required. If the parameter is required, no default value may be specified. Failure to supply a value for a required parameter gives a run-time error (the specification says that in the case of call-template, it should be a static error).

In XSLT 3.0, xsl:param can also appear as a child of xsl:iterate.

XSLT 3.0 introduces static parameters (global parameters declared with static="yes"). These are implemented in Saxon 9.5. The value of a static parameter must be known at stylesheet compile time. Static parameters can be referenced in [xsl:]use-when attributes, and also in the initializers of other static parameters and variables. The scope rules are a little different from other global variables and parameters - forwards references are not allowed. The values of static parameters may be supplied from the command line in the same way as dynamic parameters; they may also be supplied via a new method call on the s9api XsltCompiler object.