"May not" -- no choice vs. mere restriction

This is from the tabletop game Warhammer 40k rulebook:

The turn Marines arrive they may not assault.

I always assumed it means they are not allowed to assault. But the meaning "they may choose not to assault" (as opposed to "they must always assault") would be perfectly legitimate too, as far as game rules go.

So how do you tell whether it limits your choices (not allowed to) or extends them (allowed not to)? How it should be worded if the meaning was "allowed not to"?

Solution 1:

'may not' is only slightly ambiguous in general ("is not allowed" is preferred to "can choose not to"). In this context though, it definitely takes on the "is not allowed" meaning because the negative, "they may assault" was not used.

That is, though the literal meaning is ambiguous, the implication (because of not using the negation) forces the interpretation to be "is not allowed".

Solution 2:

This is a poorly worded rule. You are correct that both interpretations are possible.

Possible rewordings are:

• The turn Marines arrive they are not permitted to assault

or

• The turn Marines arrive, they may choose to assault or not

I would imagine that it is the former that is the case; rules in games (especially battle games such as this) tend to place limits on players' actions, rather than enumerate all of their options (which should be many and varied in such games).

Solution 3:

"May not" is tricky in this context because it is listed in a set of rules. You should always refer to the rulebook itself for its own uses. If the rulebook does not tell you what to do, find a relevant authority on the game itself.

The English phrase implies that you should not or cannot do whatever comes after the "may not." Rulebooks like to use the phrase "may not" to let players opt out of an action because "cannot" can take the place of "not allowed." Again, without knowing more about the game or its rules, I cannot help with answering this particular context. Feel free to ask about more specific rules interpretations at boardgames.SE or gaming.SE