Mark all unticked articles in this group as read (gnus-group-catchup-current). gnus-group-catchup-group-hook is called when catching up a group from the group buffer.

Mark all articles in this group, even the ticked ones, as read (gnus-group-catchup-current-all).

Clear the data from the current group---nix out marks and the list of read articles (gnus-group-clear-data).

If you have switched from one NNTP server to another, all your marks and read ranges have become worthless. You can use this command to clear out all data that you have on your native groups. Use with caution.