There is no restriction anywhere on the content or effect of the agreement reached under Article 50, so in principle the exit terms could be readmission, which is equivalent to aborting the process by universal agreement (it could also be to extend the term for which they are still in the EU pending exit indefinitely, with the same effect.)

Once Article 50 is triggered, there are two possibilities: either they are out with no special status in two years because no other agreement is reached before that time, or some agreement is reached within two years which gives some other terms, which terms are nowhere restricted.