Unsubscribe group in scheduled mailings
The unsubscribe group provides a way for recipients to opt out of future mailings.  It isn't possible to send a mailing without this.

Please read the documentation for more information about how this works:  Mass mailings using CiviMail

Here's an extract that provides an overview:


  For mailings to search results, you are required to choose a Group
  from the Unsubscription Group dropdown menu. Here's why: Every mass
  mailing needs a way to track unsubscribe requests. A simple
  "unsubscribe" process is required by law in many countries and may
  help prevent your mailings from being treated as spam. Mailings sent
  to Groups have this capacity built-in. The next time a mass email is
  sent to that Group, anyone who has unsubscribed will not be included.
  However, mailings sent to search results do not have this built-in way
  to track who has unsubscribed, so you need to provide one.