Some extensions may appear directly in the application. The idea is to not require users to load the website in a browser, search for an extension, download, and eventually install it (although this is always possible). At several places we offer a so-called Tight Integration, for example at the Template Manager, the Gallery, the Icon Theme options etc., where only those extension are listed in a dialog that match the tag. This tag can be assigned freely, and should be done wisely. The moderator will carefully review it.
The implementation of this feature was realized in a GSoC project and is not perfect. We need to generate a special database for the access, which is done once per day. So please be patient if your submission is not listed immediately. And some quirks exist in the application too, see tdf#133026.