SSI support - Server Side Include (or SSI) is a used to include files into web pages. When SSI Support is enabled, the web server will expect files that contain SSI directives to carry the extension .shtml. Shtml files are normal html documents. The web server only parses files with the extension .shtml for SSI directives. It is not necessary to enable If this support is not required.
More information on SSI can be found Server-side includes (SSI) (st-andrews.ac.uk)
