The <x:if> tag in JSTL (JavaServer Pages Standard Tag Library) is used to implement conditional logic based on the values of XML nodes or attributes. Here's an example of how to use it:

<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %>
<%@ taglib uri="http://java.sun.com/jsp/jstl/xml" prefix="x" %>

<x:parse var="myXml">
  <root>
    <item id="1">Item 1</item>
    <item id="2">Item 2</item>
    <item id="3">Item 3</item>
  </root>
</x:parse>


<x:forEach select="$myXml/root/item" var="item">
    <x:if select="$item/@id = '1'">
      - <b>${item}</b>

    </x:if>
    <x:if select="$item/@id = '2'">
      - <i>${item}</i>

    </x:if>
    <x:if select="$item/@id != '1' and $item/@id != '2'">
      - ${item}

    </x:if>
  </x:forEach>

In this example, we use the <x:parse> tag to parse an XML document into an object named myXml. We then use the <x:forEach> tag to iterate over the item elements in the XML document.

Within the <x:forEach> tag, we use the <x:if> tag to implement conditional logic. The select attribute is used to specify an XPath expression that tests whether the id attribute of the item element is equal to '1', '2', or neither. We use the ${item} expression to print the text content of the current item element.

Note that the <x:if> tag can also be used with other XPath expressions to test the values of other nodes or attributes. We can use logical operators such as and, or, and not to combine multiple conditions.