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 in the fmt tag library is the <fmt:setBundle> tag, which is used to set the resource bundle to use for localization.

The <fmt:setBundle> tag is used to set the resource bundle to use for localization. The resource bundle contains localized versions of messages and other data used in the JSP page. The tag should be placed at the top of the JSP page, before any data is sent to the server. The syntax of the tag is as follows:

<fmt:setBundle basename="resourceBundleName" [var="varName"] />

where resourceBundleName is the name of the resource bundle to use, and varName is the name of the variable to store the resource bundle.

For example, to set the resource bundle to use for localization to messages, you would use the following tag:

<fmt:setBundle basename="messages" />

This tag sets the resource bundle to use for localization to the messages bundle.

You can also specify a variable to store the resource bundle, like this:

<fmt:setBundle basename="messages" var="bundle" />

This tag sets the resource bundle to use for localization to the messages bundle and stores it in the variable bundle.

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.