[This is preliminary documentation and subject to change]
You can use server-side include (SSI) directives to include text, graphics, or application content into a Web page just before sending that content to the browser. SSI can be used to include, for example, a time/date stamp, a copyright notice, or a form for a customer to fill out and return. Including a file is an easy way to incorporate text or graphics that are repeated in many files. Rather than typing the content into every file, you save the content into an include file. The include files is called through a simple statement that instructs the Web server to insert the content into applicable Web pages. Furthermore, you need only to change the content in one place when you use include files.
Because a file containing SSI directives requires special processing, you must give any SSI files an SSI file name extension. The default extensions are .stm, .shtm, and .shtml. You can add additional extensions to the list recognized by the Web server; see Setting Application Mappings.
The Web server processes SSI directives while it is processes the Web page. When the Web server reaches an SSI directive, it inserts the contents of the include file directly into the HTML page. And if the include file contains an SSI directive, that file is also inserted. In addition to the basic directive used to include a file, you can use SSI directives to insert information about a file (such as its size) or to run an application or a shell command.
For more information, see the Server-Side Includes Reference.