RAW (Rules As Written)
RAD (Rules As Designed) -- NOTE: this replaces Rules As Intended because of the conflict with the next one.
RAI (Rules As Interpreted) -- NOTE: the implication here is for interpreting based on a guess or personal bias.
It will get long after the break and since I'm not writing something for academics those who have some knowledge of this subject might note a lack of rigor, but then I'm aiming this more to the majority I read on the Internet who don't get the context..
RAD is the next most authoritative and means interpreting an ambiguous rule on the basis of what the writer's design or intent was. Now GW really sux at doing this since they don't give designers' notes or even respond in a useful manner to rules inquires. Not to be dismissed though, because sometimes the design intent can be clearly discerned.
RAI is the last, it covers the reasonable inferences that can be drawn from other rules all the way to blatant misinterpretations of the rules. It still can have some validity, but usually it means the facile interpretation. This is where the most rigor in logic needs to be applied and so often isn't. "Common consensus" is often appealed to, but if your mother ever asked you, "Well if everyone else was jumping off a cliff would you jump too?", then you should know about how well that can work.
How can we tell which is which and at the critical category of RAI, how can we decide how to interpret? When is focusing on a few words or a phrase being hyper-literal and when is it simply pointing out the key to the issue?
Starting with some basics, the first thing you must assume is that the text you are reading is meant to be read in the most straightforward manner possible and that the dictionary meanings of words are used unless there is a glossary definition. An example of error in when people try to figure some difference between "removed from the board" and "removed as a casualty" and do so by trying to put extra meaning onto the word "casualty" where it is mentioned and narrowly redefine it by "extracting" the meaning from some mention of it. A common place was in the Sweeping Advance section, which supposedly weren't casualties, even though the description given in Sweeping Advance directly fit the dictionary definition of "casualty" as used in the military sense. Oh, wait, I just narrowed the definition down to the "military sense". Yes, that has to be justified, which I will do so by pointing out that the discussion is about a wargame, so that would imply that "casualty" would use the military subdefinition. It appears that GW noticed that the illegitimate redefinition of casualty away from the dictionary definition was allowing for certain misinterpretations of rules to occur and in this case added "as casualties" to Sweeping Advance in 6th.
The two major ways to overrule a simple straight reading of the text are to show a contradictory text which needs to be resolved or to take the text as literally as possible and see if there is an error. For example, there was a blatant misinterpretation of the rules for the Necron Night Scythe with lots of people making up lots of reasons to first prove then any units "embarked" on the NS are forced to "take the hit" if the flyer is shot down before they can go into reserves (sometimes with the corollary that, oops, they aren't on the table, no RP roll for you). This fails both tests since no matter what, the obvious intent just from the rules was that the unit was supposed to avoid such a fate. In the first way, a simple reading of the unit entry for the NS has an entire paragraph about how the design of the worm hole gate was specifically to allow the unit to avoid the fate of passengers that were really inside their transports. Oh, did I use FLUFF to justify that? FLUFF never justifies anything, does it. Wrong. Fluff sometimes can be used to discern the designed intent of the actual rules, in this case using RAD to support RAW despite the fantasies of some. Not always and not very often if you consider in the generic background fluff. Even in unit fluff there are useless bits, the Necrons are full of references to the "fear factor" many of their units provoke, and that was originally part of the Necrons, but the new codex left in the references and cut out the rules. However, there is a big difference between fluff that explains wargear on the books and using fluff to make up rules that GW did not include.
The second way the try to make the NS passengers "take the hit" fails comes from its dependence on structuring some sequence of simultaneous events (and amusingly enough trying to justify it with imaginary "fluff" about how it happens) and an overly literal reading of the rules. The counter is to read the rules even more literally, which I did by pointing out that I didn't care if the Necron passengers "took the hit", because the BRB says that any passengers that cannot be placed in a certain area are destroyed. Since any surviving passengers from a crashed NS are placed in reserve, they cannot be placed on the board in a certain area and they are destroyed. Stupid? Absolutely and the hue and cry and whining was immense, but it was nothing more than following the methodology of the people who were claiming that the passengers had to take the hit were using to justify their position.
Here I might mention Occam's Razor, which I often see misused to support the most facile of two positions because it is "simpler" even though it has to make the most assumptions. Often that is because breaking down the issues and describing each step of the logic chain in the position that uses less assumptions gets dismissed by many (but not all) as "too complicated" and "wall of text". I'll leave it to the reader to decide what that says about people trying to judge between two decisions.
I'll also note here that the claim "GW writes bad rules" should raise a red flag when deciding between two positions. If both agree that the rules are badly written, there is a good chance that they are right, but if only one side is making that claim, then they are most likely pushing a bad interpretation.
Okay, this is incomplete, but it is getting long enough. I'll try and pick it up a bit later with another post.
Always consider the rules are written as they are intended first and read them straight.
Follow any interpretations to their logical conclusions and see if anything weird comes up.
Check to see if there are any rules that modify the reading.
Read the entire context.
If one side is claiming justification on the basis of GW writes bad rules, they are probably wrong.