Implementing IF-ELSE condition in ItemStyle.xsl file in SharePoint
In XSLT, there is no IF-ELS condition. However, you can use choose statement to perform IF-ELSE and NESTED IF in XSLT as the following:
<xsl:choose>
    <xsl:when test="">
       <!--do something-->
   </xsl:when>
   <xsl:otherwise>
       <!-- Else do somthing -->
    </xsl:otherwise>
</xsl:choose>
Implementing NESTED-IF-ELSE in SharePoint ItemStyle.xsl file
If you would like to implement multiple IF in ItemStyle.xsl file, you have to repeat when clause as the following:
<xsl:choose>
    <xsl:when test="">
       <!--do something-->
   </xsl:when>
    <xsl:when test="">
       <!--do something-->
   </xsl:when>
    <xsl:when test="">
       <!--do something-->
   </xsl:when>
   <xsl:otherwise>
       <!-- Else do somthing -->
    </xsl:otherwise>
</xsl:choose>
Implementing NESTED CHOOSE in SharePoint ItemStyle.xsl file
Besides the above examples, you can also use NESTED CHOOSE in XSLT as the following:
<xsl:choose>
    <xsl:when test="">
         <xsl:choose>
           <xsl:when test="">
            <!--do something-->
            </xsl:when>
             <xsl:otherwise>
             <!-- Else do somthing -->
           </xsl:otherwise>
         </xsl:choose>
   </xsl:when>
    <xsl:when test="">
       <!--do something-->
   </xsl:when>
    <xsl:when test="">
       <!--do something-->
   </xsl:when>
   <xsl:otherwise>
       <!-- Else do somthing -->
    </xsl:otherwise>
</xsl:choose>
NESTED-IF and Multiple CHOOSE in XSLT
Below is an example to perform Nested-IF and Multiple CHOOSE in XSLT ItemStyle file in SharePoint 
