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. Another commonly used tag in the fmt tag library is the <fmt:formatNumber> tag, which is used for formatting numeric values.

The <fmt:formatNumber> tag accepts a numeric value as input and formats it according to the specified pattern. The syntax of the tag is as follows:

<fmt:formatNumber value="numericValue" pattern="numericFormatPattern" />

where numericValue is the numeric value to be formatted and numericFormatPattern is the pattern used to format the numeric value. The pattern attribute specifies the format of the output string, using a combination of letters and symbols that represent different parts of the number.

For example, to format the number 12345.6789 in the format "#,##0.00", you would use the following tag:

<fmt:formatNumber value="12345.6789" pattern="#,##0.00" />

The output of the tag would be a string representing the number 12345.6789 in the specified format.

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.