This is an extension of the standard SSI commands available in the XSSI module, which became a standard part of the Apache distribution in Version 1.2. XSSI adds the following abilities to the standard SSI:
XSSI allows variables in any SSI commands. For example, the last modification time of the current document could be obtained with:
<tt><!--#flastmod file="$DOCUMENT_NAME" -->.
The set command sets variables within the SSI.
The SSI commands if, else, elif, and endif are used to include parts of the file based on conditional tests. For example, the $HTTP_USER_AGENT variable could be tested to see the type of browser, and different HTML codes output depending on the browser capabilities.