In the interest of forging the narrative and through a step-by-step playthrough of the shooting phase, I believe it is unambiguous that Jink must be declared before Markerlights.
Tau player enters the shooting phase.
Tau elects a unit to attack with the Markerlight, a ranged weapon with the special rule "Target Acquired". This attack may not be saved against (p.68).
Tau player might say, as an example, "this unit is attacking your jetbikes with my markerlights"
Tau player rolls and completes the attack(s) applying markerlight tokens to the jetbike unit.
Tau player declares a new shooting attack against the markerlit unit.
Tau player might say, "this unit is attacking your jetbikes with these plasma weapons."
Tau player asks if the defending player has any response
Tau player, having given the defending player a chance to declare jink, can then declare usage of markerlights and how they are spent specifically.
Tau player may then make to-hit rolls and resolve the attack.
Some may point out that per the jink special rule, targeting occurs then opting for jink, then declaration of weapons. In my experience, games never break up attacks into individual steps, asking for acknowledgement after each step; it is more natural (and time-saving...and fun) to make more complete declarations--always giving no less than the amount of information required at that point in time.
To put in a more concrete example, the Tau player does not say "this unit is attacking your jetbikes with these plasma weapons and I'm expending 2 markerlights for Scour (ignores cover)". The Tau player simply makes his intent clear while giving an opportunity for the defender to make his tactical response.
Nor does the Tau player say "this unit is attacking your jetbikes, do you jink?" The Tau player consolidates the first 3 steps of Nominate, Target Unit, Select Weapon.
The fact that declaring the plasma weapons (step 3 of the Shooting Sequence, BRB) occurred at the same time as step 1 and 2 may be technically "out of order", but the attacking player doing so does not put the defending player at a disadvantage by doing so. On the contrary, the defending player now has more information on which he can decide whether or not he wishes to jink, while still retaining the option to jink or not, because Jink simply must just occur before To-Hit rolls.
Forging the Narrative
A unit of jetbikes has been markerlit. Then, the markerlit squad
recognizes plasma weapons from a distance unit re-orienting and
pointing right at them.
In response to the incipient barrage of
plasma, the Jetbikes make a split decision to Jink or not to Jink.
Seeing that they jinked (or didn't jink), the plasma
cannons--exploiting this new advantage--Pinpoints, Scours, and/or
Seeks and completes their attack.
Does the attacking squad with plasma weapons call out "Hey, we're ignoring your cover!"? No, it doesn't seem likely that they literally could know; thus, they must make their decision which happens before shots are fired.
Since 40k emphasizes Forging the Narrative so much, it seems apparent that from FTN standpoint, Jinking must occur before Markerlights.
Even if one were to laboriously go through each step of the rules with each player having their turn, in turn, Jink optioning occurs before markerlight expending.
- Tau enters the shooting phase
- Tau elects a unit to make attack with, waits for acknowlegment
- Tau elects a unit to attack, waits for acknowlegment
- Defender decides whether to jink
- Tau selects a weapon, waits for acknowlegment
- Tau expends markerlights
- Tau makes to-hit rolls.
Going through the shooting sequence one at a time still ends up with the same result: markerlights can be spent knowing whether the defender jinked or not.
Rule as written, it almost would be more clear had the rulebook left out one sentence, as it adds very little to the clarity of the intended rule.
When a unit with any models with the jink special rule is selected as
a target for a shooting attack, you may declare that it will jink.
decision must be made before any of the to hit rolls have been made.
If the unit jinks all models in the unit with this special rule gain a
4+ cover save until the start of their next movement phase, but they
can only fire snap shots until the end of their next turn.
The crossed-out line is redundant (as "selected as a target" is always before to-hit rolls), but adds the ambiguity (to some) that Jinking can happen any moment up until the dice hit the table. This does not seem to be the case: the timing is exact, after the target has been selected and before the Tau has even selected a weapon. And if defenders choose to Jink before the weapon has been selected, then certainly markerlights come after Jink, which happens immediately before firing.
Or put another way:
- Action must happen at step 2. (target)
- Action must happen before step 4. (rolling dice)
They are not contradictory, but the latter is unnecessary.
If any of this is not convincing enough, the BRB states:
While playing Warhammer 40,000, you’ll occasionally find that two or
more rules are to be resolved at the same time – normally ‘at the
start of the Movement phase’ or similar. When this happens, and the
wording is not explicit as to which rule is resolved first, then the
player whose turn it is chooses the order.
In this case the dispute is about "immediately before shooting" and "before to-hit rolls are made." In the event the players agree that both should be resolved at the same time, but it is uncertain which would then be resolved first, the Tau player may choose the order. To his best advantage, "before to-hit rolls are made" can be chosen to resolve first, then "immediately before shooting"--once again allowing the expending of Markerlights to occur after Jink optioning.
If the players do not agree they should be resolved simultaneously (and be subject to the acting player's whim), the players involved should roll-off to settle rule-disputes.