All the following marks mark articles as read.
gnus-sparse-mark). See Customizing Threading.
gnus-duplicate-mark). See Duplicate Suppression.
All these marks just mean that the article is marked as read, really. They are interpreted differently when doing adaptive scoring, though.
One more special mark, though:
Marking articles as expirable (or have them marked as such automatically) doesn't make much sense in normal groups—a user doesn't control expiring of news articles, but in mail groups, for instance, articles marked as expirable can be deleted by Gnus at any time.