The configuration tab of your subsite holds settings used by the site frameworks and themes.
General Settings
Most subsites have the same common options that are used across themes. For example, most sites will have a homepage, a login article and a 404 message. You'll also find settings for Google Analytics, Tag Manager, and Maps, a robots.txt file, and the "from" address used in emails sent out by the site.
The options you see here will vary depending on the version of the site frameworks your subsite uses. You can see all of the options in the Subsite Configuration - Framework Features section.
Theme Settings
Subsite themes each have they own configuration options. The second section of this page will change depending on the theme you select for your site.
Each of our standard themes is looked at in more detail in the Theme Documentation section.
Technical Note
The tab is enabled on the Advanced Settings section of Autoconfig by entering the name of the private form that should be displayed. The standard form is called SUBSITEEXTRAS.