The Extensions administration region is not an out-of-the-box feature. It appears to the higher user roles if any of the admin-side plugins that employ it are installed.
When relevant, the Extensions administration region will be seen in the administration navigation from all admin-side views except when you are in context of the Plugins panel, even for the administrator. This behaviour ensures being able to safely deactivate and/or remove outdated or buggy plugins if they are causing problems. To access the Extensions administration region from the Plugins panel, move to anywhere else in the admin-side first, and the Extensions administration region will appear in the navigation again.
When you upload a plugin that uses the Extensions administration region, a new panel option will appear under the region that provides the plugin’s associated functionality or configuration controls.
Unconventional panel placement
The use of the Extensions administration region is a convention adopted by plugin developers to put administration-side plugin preferences and other functional controls in a logical place, and that’s a good thing when it can be done.
Not all administration-side plugins use the Extensions administration region, however. Some plugins will add their associated admin-side panel in one of the core regions. This may happen when panel placement is decided with respect to functional context over convention. Another reason may be a concern for having too many plugin panels under the Extensions administration region, though this is likely a moot issue for most Textpattern users.