I think you are talking about The Division 2. If so, that forum is here: https://forums.ubisoft.com/forumdisp...ral-Discussion
If you are indeed talking about TD1, then the only restrictions I know of here are by character level. You can be max level in the open world for instance, but will still be locked out of harder missions in The Underground, as well as certain directives will be unavailable until at a higher UG level. The same goes for higher difficulties in the open world.