This command allows you to include a file into your html code. The contents of the file are loaded in at the exact spot where the <!--#include virtual="../ssi/file.ssi" --> is.

This is an example of relative path.

As you can see I like to put all my ssi stuff into a folder called ssi. That way all the includes are in one easy spot to find for update.

Also you will notice that I have chosen to call my included files .ssi

This is your choice. There is no requirement.