Can an ABD (all but dissertation) return to the university to redo the dissertation? If yes, how do they proceed?
ABD isn't usually thought of as a final status. Most people don't leave their university ABD. Most people spend a couple of years in the program, having passed quals, taken all the classes, and proposed a topic working on their dissertations. At this point, they are colloquially considered "All But Dissertation" because that describes what they've done so far not an official status. Every university will have a policy about what happens if you leave while ABD, and most will have some mechanism for coming back in reasonable time (not decades later) and finishing. Ask your advisor and graduate school or coordinator.
tl;dr: it depends
We had two students who were AWOL for many years while ABD and tried to submit:
The first student had delayed writing for a decade or so due to family commitments but kept in regular touch with faculty, refreshed the theory and data of the dissertation, reconstituted a committee, submitted the dissertation, and graduated.
The second student had not kept in touch with faculty. Their primary advisor had passed away many years ago and they had no committee. The student asked the department whether they could submit their dissertation but the data and theory used in the dissertation was decades old. They were not allowed to submit the dissertation.
Whether or not you will be allowed to even submit a dissertation depends on your university, on the provost, and then finally on the department and advisor. You will need to reconstitute a dissertation committee. After submission, the department will then have to decide whether to accept the dissertation as proof of your qualification for the PhD.
Yes, every university has its own specific policy. Some policies may or may not include:
- A person who has completed their classes and has passed qualifying exams becomes a doctoral candidate, and is, colloquially, ABD.
- ABD status at some universities may also be called a Candidate in Philosophy.
- Some schools confer the degree of MPhil to recognize a candidate's accomplishments until a candidate completes and defends their research, at which point, if successful, a candidate earns a Ph.D. or other doctoral degree.
- At some universities, there are some very specific ABD Status Agreements, including considerations for status as ABD In Residence, and ABD In Absentia.
- At some universities, you are ABD only so long as you have not abandoned your degree program and have not timed-out.
- At some universities, you are no longer, by their policy, ABD if you have left your degree program, or you have timed out.
It's important to recognize that more often than not "ABD" is not a credential conferred, granted, or bestowed. Rather "ABD" is an indication of status--that status being you have not yet completed your research, and/or defended your research.
Generally speaking, you should check with your university to understand their policy regarding the status, "ABD."