index.cgi

This is a python script written for cgi.
By default a2hosting created a cgi-bin directory when I created the subdomain, but the script runs from any folder.

Directory Index

By default, with no .htaccess directive, the default page for a folder is defined as
DirectoryIndex index.php index.cgi index.html

Filename Suffix

The suffix lets Apache know how to preprocess the file.

index.cgi
By default a2hosting created a cgi-bin directory when I created the subdomain, but the script runs from any folder.
index.php
This file is suffixed with .php, so php sections are interpreted.
Most likely this is implemented with mod_php which allows for mixed html and php.
index.html
Straight HMTL. PHP sections are ignored.