In the normal course of racing / scoring, a boat that reports their retirement (either before or after finishing) would be scored RET.

If they do not report, and do not finish, then they would be scored DNF.  However, a boat that doesn't finish and just disappears from the race course, should immediately trigger a "What happened to them?" question and subsequent investigation by the RC - thus the (possible) imposition of a penalty for not reporting.

There is no difference in the points given between the two scoring codes, unless changed by the sailing instructions.  If there's no difference in the points, it really doesn't matter what code you use.

Thank you Matt for your prompt and clear answer. That was my guess too (but of course I have my good reasons to ask, even if I cannot disclose the details as per this Forum's rules)

With regard to your final statement ("If there's no difference in the points, it really doesn't matter what code you use.") I am not 100% convinced that it doesn't really matter...
I mean, a similar thought may apply to other two similar [but not identical] scores, DNC and DNS
In fact, some software does include only one of them
But although there is no difference in points between DNC and DNS, their meaning is inherently different. As per DNF vs RET ... in my opinion :-)

For me the answers are:

1a - DNF
1b - DNF

2a-d - It depends on whether the boat has finished the race or not prior to retiring.

I think there are some subtle differences that often get overlooked on this because the points are generally the same, but the definitions are in Appendix A.  I think the rules themselves add to the confusion by using the word 'retire' in rules like 44.1(b).  In 44.1(b) the 'retirement' could happen before or after finishing, but that is different whether her score is DNF or RET.

In longer series there is a difference (RRS A9) between DNC (score = number of boats entered+1) and DNS (score = number of starters+1) as there is some reward for making the effort to race.  I don't know if there are situations where DNF and RET would be scored differently.

Thank you John for your thorough reply.
Definitely you are right : DNC and DNS do have a different impact in longer series (that is why, on another venue, I opened a discussion also on that, because the software which was in use didn't differentiate the two)

However, about your expanded definition of RET, I believe that that is more suitable to the old scoring RAF (Retired After Finishing)...
RAF was substituted by RET in 2013 (if I'm not wrong) purposely for including a retirement before finishing.  
But I am the rookie here, and I might very well be wrong...

I may just be old school on this but from a scoring perspective you may need to distinguish between boats that actually finished and ones that didn't, for example, when considering or determining redress. The rules writers may have had a different opinion.  Otherwise, the only time the RC would score a boat DNF would be if they fail to finish based on a-c in the definition of finish.  Maybe that's what they were after.

Thank you John for your clarification

Daniele .. just a quick note that I've been thinking about you and all our Italian RRoS forum-friends these past weeks.  I hope things start to turn a corner for you all quickly! - Ang

Thank you Angelo
It has been very kind of you :-)

I have been scorer for a large distance race and use of RET to indicate proper withdrawal is important to know when invitations are sent out for the following year. 

In the 2013-2016 rule book submission 184-11 changed RAF (Retired After Finishing) to RET (Retired). The stated purpose and reason are below. Based upon this submission I have interpreted RET to mean that a boat is retiring as required by rule 44.1(b)) or by Appendix P2.2.

Submission 184-11
Purpose or Objective
"To provide correct scoring abbreviations for boats that (1) take a penalty by retiring during a race, or (2) are penalized when the sailing instructions provide for a penalty other than disqualification."

Reason
"At present, there is no scoring abbreviation for a boat that takes a penalty by retiring during a race (rule 44.1(b)) or retires as required by Appendix P2.2 or P2.3. In these circumstances, the race committee either scores the boat RAF or DNF. RAF is incorrect, since the boat retired before finishing. DNF does not adequately describe the situation.

RET is the appropriate abbreviation to use for a boat that retires in these circumstances. There is no reason to maintain RAF, since RET covers any retirement, whether before or after finishing.

Rule 64.1 permits the sailing instructions to specify a penalty other than disqualification in some circumstances. There is no scoring abbreviation in A11 to describe such a penalty. DPI has gained near-universal acceptance. It is recommended, therefore, that this abbreviation be added to A11."

RULE REFERENCES TO RETIRE

Racing A boat is racing from her preparatory signal until she finishes and clears the finishing line and marks or retires, or until the race committee signals a general recall, postponement or abandonment.

SPORTSMANSHIP AND THE RULES - Competitors in the sport of sailing are governed by a body of rules that they are expected to follow and enforce. A fundamental principle of sportsmanship is that when competitors break a rule they will promptly take a penalty, which may be to retire.

44.1 (b) if the boat caused injury or serious damage or, despite taking a penalty, gained a significant advantage in the race or series by her breach her penalty shall be to retire.

90.3 Scoring
(a) The race committee shall score a race or series as provided in Appendix A using the Low Point System, unless the notice of race or sailing instructions specify some other system. A race shall be scored if it is not abandoned and if one boat sails the course in compliance with rule 28 and finishes within the time limit, if any, even if she retires after finishing or is disqualified.

A4.2 A boat that did not start, did not finish, retired or was disqualified shall be scored points for the finishing place one more than the number of boats entered in the series. A boat that is penalized under rule 30.2 or that takes a penalty under rule 44.3(a) shall be scored points as provided in rule 44.3(c).

A5  SCORES DETERMINED BY THE RACE COMMITTEE
A boat that did not start or finish or comply with rule 30.2, 30.3, 30.4 or 78.2, or that retires or takes a penalty under rule 44.3(a), shall be scored accordingly by the race committee without a hearing. Only the protest committee may take other scoring actions that worsen a boat’s score.

A6.1 If a boat is disqualified from a race or retires after finishing, each boat with a worse finishing place shall be moved up one place.

A9 RACE SCORES IN A SERIES LONGER THAN A REGATTA
For a series that is held over a period of time longer than a regatta, a boat that came to the starting area but did not start, did not finish, retired or was disqualified shall be scored points for the finishing place one more than the number of boats that came to the starting area. A boat that did not come to the starting area shall be scored points for the finishing place one more than the number of boats entered in the series.

A11 SCORING ABBREVIATIONS
RET Retired

P2.2 Second Penalty
When a boat is penalized a second time during the regatta, she shall promptly retire. If she fails to do so she shall be disqualified without a hearing and her score shall not be excluded.

P2.3 Third and Subsequent Penalties
When a boat is penalized a third or subsequent time during the regatta, she shall promptly retire. If she does so her penalty shall be disqualification without a hearing and her score shall not be excluded. If she fails to do so her penalty shall be disqualification without a hearing from all races in the regatta, with no score excluded, and the protest committee shall consider calling a hearing under rule 69.2.

Thank you Chuck :-)

Thank you Mark. That's an excellent insight.
And that explains why those who are more senior than me (and witnessed the introduction of the abbreviation RET in 2013) are so firm in stating that RET only applies to boats who retire as a consequence of them breaching a rule. 
Now it makes much more sense to me ! :-)

One additional thought on the subject. If retired (RET) is intended to be the scoring abbreviation that denotes when a competitor broke a rule. Then the sailing instructions might be more accurately written:

A boat that does not intend to finish a race, or retires from a race after breaking a rule, shall notify the race committee as soon as possible. [DP]

Maybe the subject for another thread!!

Thank you :-)
I saw also SIs where there are rules to the boats who retire for completing a Retirement Declaration, or signing a Retirement Form. Usually those instructions have the [DP] notation.

Based on the definition of racing, a boat which retires is no longer racing. A boat that does not finish is still technically racing. Might make a difference if such a boat was involved in an incident with another boat. 

That said, I would say that a boat is retired from the point that the person in charge has decided to retire, not necessarily from thettime she informs somebody else.