XSSI include directive
The include directive inserts the contents of another file into the current file.

Basic syntax:
<!--#include virtual="some-URL" -->

How to insert a new include directive
To include the contents of a file at the cursor's current location in your document, either choose "XSSI - include file" from the Insert menu or click the icon in the XSSI category on the floating Object palette. Simply type the URL for the file you want to include or click the yellow folder icon to browse for a file. You can also choose an XSSI variable from the list if you've previously assigned a URL to a variable. Then click "OK" to insert the directive. If you have automatic translation enabled (or you manually translate your document), the included file will be displayed. Otherwise, an icon will appear to show you that there is an invisible element on your page.

Note: all variables that are set in the document will appear in the list. In addition, if any environment variables have been set using the Set Environment Variables command, they will appear in the list as well.

How to change an existing include directive
To change an existing include directive, select the directive either by clicking the included text (if it's been translated) or invisible element icon or by clicking within the directive in the HTML inspector window. When you do this, the Property Inspector for XSSI include directives should appear. It looks like this:

Here, you can change the file to be included, the same way you created it.