The contribution setting is set in admin->contribution. A contribution can be mandatory or not in a per-feature basis.
These contributions are displayed in page history and, if chosen, in the comment or post.
Another setting related to contributions is at Admin -> Login ("Displays user's contribution in the user information page:"). If enabled, a user will be able to see other user's activity log, including his/her contributions on the report. For 4.x this setting moved to second tab of Admin -> Community.