These are all users registered in the website, both in the backend and in the frontend.
In Connect this list to dropdown box, select the membership provider for the users which you want to add as subscribers.
The dropdown box contains all created membership providers.
Sitefinity forms module
These are the responses submitted to a form on your website. For more information, see Creating and editing forms.
In Connect this list to dropdown box, select the form, which responses you want to use as subscribers.