Actually it just occurred to me that there may be an easier answer. A skill which fails its Requirements becomes an Idle. BS Attack is a skill. A subset of the Burst from a BS Attack isn't a skill, and can't become an Idle. I can't declare shots split between A and B, then have the shots against A succeed and against B become an Idle. Either I perform BS Attack, or I perform Idle. So, if I can split my burst in that way, then the only possible outcome would be that the whole BS Attack is converted to an Idle, so my shots fail against my intended target as well as my fake target. Which would defeat the point, and make the question moot.
https://infinitythewiki.com/Combat_Module_Intro#Burst_.28B.29 Note that when that rule was written all Requirements were checked during the Resolution of the Order.
It's a skill the Requirements of which has not been met when those Requirements are required to be fulfilled so it's resolved as an Idle. That's how the rules technically handle that. See above for where I cite the rules that explain how it's handled in the case of Burst.
Ah, good point @inane.imp . I checked the BS Attack rule but didn't think to check the separate Burst rule. That does suggest that it's possible to assign Burst in ways that don't meet the Requirements. And since the only Requirements of BS Attack are to be using a BS weapon, have LoF, and not start the order Engaged, it would certainly seem that you must be able to assign Burst to a target outside LoF, that being the only way that some dice could wind up not meeting the Requirements. Unless that whole rule is just obsolete as a result of the FAQ, which is possible.
You also assign Burst as the result of skills that have other Requirements. My point is simply that the FAQ did not establish a prerequisite that needs to met prior to declaring a LOF based skill. Rather it shifted the requirement to check whether Requirements are met from Resolution to Declaration for LOF-based Requirements. That's it. So how you perform a Requirement check is still unchanged and, importantly in this context, the fact that you can fail a Requirements check is still a factor. And, to be absolutely clear, the concept of a Requirement check is in the Rules: "Resolution: Check that the declared Skills, Special Skills, and pieces of Equipment meet their respective Requirements,". For whatever reason, in the FAQ, CB uses the word "fulfilled" rather than "met" but functionally they're the same word in this context. We unequivocally know what happens to a skills the Requirements of which are not "met" so I have NFI why we're arguing about what happens to a skill the Requirements of which are not "fulfilled".
If you're right that the non-LoF burst can be declared, then it doesn't even get resolved as an idle - the dice are just "lost" per the rule you quoted. So you declare some dice against an invalid target, then at Resolution you go "yup, as we all knew, those shots were invalid when I declared them, so I lose those dice and will roll the rest." It's at least a plausible interpretation. There's still a lot of N3 instinct that says "you can't declare things you know are invalid." We saw it in the weird knee-jerk internet reaction when ijw confirmed that you can declare Dodge even when obviously outside ZoC. But in that case, you don't technically know you're outside ZoC. In the present case, the rules provide explicitly that we know we don't have LoF at declaration, so N3 thinking still says we can't declare. Maybe you're right that in N4, we can.
Yeah, as if "Even though I just said that you have to satisfy the line of fire requirements for BS Attacks when you declare the attack, I didn't actually mean it" is what IJW meant when posting that you can't declare an invalid BS Attack. Seriously. Simply because for every BS Attack, you have at least one burst value to assign, and there's no valid argument that a BS Attack with B1 works differently than a BS Attack with B2.
Is there a link for that quote? If he said "you can't declare an invalid BS attack," that could be a definitive answer to the question, for sure.
His interim ruling is even more clear that it's just a timing change on when you check requirements not something that prevents declaration.
If we assume that he chose his words extremely carefully, then I agree with you. The question is what exactly the word "fulfilled" means in the FAQ - does it mean that the Requirement has to be "fulfilled" in order to declare the skill, or does "fulfilled" refer to the process of checking the Requirement and converting to Idle if it fails? In his Interim Ruling, ijw said this: Implications - Skills are not restricted by the situation when the Skill is declared, so a Skill declaration can either: Be fulfilled during the Order when you check the Requirements in the Resolution step, Not be fulfilled, and the Skill becomes an Idle. (https://forum.corvusbelli.com/threa...-skill-requirements-cc-aro-baiting-etc.39477/) So he was certainly using "fulfill" to mean "check the Requirement, and if not met, convert the skill to Idle." If he was being careful to use the word in the same sense that it's used in the FAQ, then I think it would follow that you can indeed declare a BS Attack (or assign some of the burst of a BS Attack) against a target outside LoF. My only proviso would be that we don't know whether he intended the interim ruling to define "fulfilled" or to use it consistently. Given that the purpose of the interim ruling was to patch CC Attack declaration, not BS Attack. In any case, it seems he hasn't said the reverse either, or at least the poster who claimed that he had hasn't provided a link.