The JSTL (JavaServer Pages Standard Tag Library) fmt tag library provides a set of tags for formatting and internationalizing data in JSP (JavaServer Pages) pages. One of the tags that can be used in conjunction with the <fmt:message> tag for formatting messages with variable arguments is the <fmt:param> tag.

The <fmt:param> tag is used to specify a parameter value for a message. It can be used within the <fmt:message> tag to pass values to the message's placeholders. The syntax of the tag is as follows:

<fmt:param value="paramValue" [var="varName"] />

where paramValue is the value to be passed as a parameter to the message, and varName is the name of the variable to store the formatted parameter value.

For example, to format a message that includes a variable argument for a user's name, you could use the following code:

<fmt:message key="welcome.user">
  <fmt:param value="${user.firstName}" />
  <fmt:param value="${user.lastName}" />
</fmt:message>

In this example, the <fmt:message> tag retrieves the message with the key "welcome.user" and uses the <fmt:param> tags to specify the values of the placeholders in the message. The formatted message is then displayed on the page.

The value attribute of the <fmt:param> tag can be any expression that evaluates to a value that can be formatted as a string. The var attribute can be used to store the formatted value in a variable for later use.

Note that the fmt tag library must be imported at the beginning of the JSP page using the following tag:

<%@ taglib prefix="fmt" uri="http://java.sun.com/jsp/jstl/fmt" %>

This imports the fmt tag library and assigns it a prefix of fmt.