Had this notion come up several times in rules conversations over the past few days, and it has inspired me for a quick BLOG post and likely a little conversation on the next podcast.
There exists a fantastic list of canons which you not only can but absolutely should use when trying to make correct rules decisions for 40K. The reason why you have to have a set of canons rather than a singular principle/canon (principle such as: Read as Written, a principle which fails at face value and is easy to show why) is because the rule set, as we all very well know, lacks clarity, structure, and technical writing which is otherwise required.
In the presence of rules (laws) which lack clarity, if you want to arrive at correct conclusions when trying to rules arbitrate, you must apply an entire set of canons, one of which is Read as Written. Another, and far more important in the land of rules arbitration, is the Right to Rules canon.
Right to Rules - When making a rules decision, rule in favor of a model/unit having a right to use the rules it was given.
Sounds stupidly obvious, right? A model or unit has a basic right to use the rules that it was given. Derp?
Well, this issue comes up quite a bit when someone is trying to over-apply the Read as Written canon to rules arbitration.
I'm not going to use any of the specific examples that have come up (three in just two days) recently as I don't want anyone to feel like I'm picking on them. Ineed, none of these issues were actually arguments anyways, just points of fact. So, instead, here's a funny one that comes up regarding everyone's favorite Pylon Star.
Technically, in the rule book, you can only apply wounds from the wound pool to the target of a shooting attack. However, as many tournaments have ruled that Death Rays (old death rays mind you) do not target models, thus conveniently ignoring Jink rules (another RAW application for another time), then according to the rules, the Death Ray cannot actually apply wounds to anything.
So, Read as Written, the Death Ray simply doesn't work at all. Case closed!
And this is why Right to Rules is important and a more powerful canon than Read as Written. Read as Written, the game actually can't be played at all due to above cases. So, you must apply Right to Rules when making a rules arbitration that no interpretation of the Rules should end up in a model or unit not having the ability to use the rules it was granted.