Note: this is only my own interpretation of how assist damage works. I could very well be wrong, and I would appreciate corrections.
Assist damage is possibly the most annoying statistic to farm IMO, not only because you are at the mercy of your teammates to actually shoot at targets that you want to be shot at, but because of the convoluted mechanics that will force you to intentionally go out of your way in order to get credit.
There are 3 types of assist damage: designated damage, de-tracking damage, and spotting damage. They are not all weighed equally however, each of these 3 types follow a hierarchy for the purpose of calculating assist damage. The hierarchy goes like this: designated damage > de-tracking damage > spotting damage. It is impossible to get credit for more than one type of assist damage at the same time. Fortunately, knowing the mechanics mean you can somewhat easily manipulate it into your favor.
As assist damage follows the strict hierarchy, it also means only 1 player can be credited with assist damage for any target. If a target is de-tracked by someone else after you have spotted it, then the assist damage will not be credited to you, but instead whoever de-tracked the target. Likewise, if you designate an already de-tracked target, you will be credited with all the assist damage until the designation wears off.
It's probably easier to explain this in point form:
- Player A spots a target. Nothing else happens. Player A gets credit.
- Player A spots a target. Player B de-tracks the same target. Player B gets credit.
- Player A spots a target. Player B designates the same target. Player B gets credit.
- Player A designates a target. Player B de-tracks the same target. Player A gets credit.
Regarding spotting damage, the game only considers EXCLUSIVE spotting damage to count towards your assist damage. That is, a target must be only spotted by you and nobody else (including infantry) in order for you to get credit. I shall summon the power of MS Paint to illustrate how this works.
In this horribly drawn diagram, you are the only one close enough to spot the target. Your ally can also see the target thanks to you, but he's not close enough to spot the target. All damage inflicted on the target by your allies will be credited to you as assist damage, as long the target isn't de-tracked or designated by someone else.
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In this equally horribly drawn diagram, both you and your ally are able to directly spot the target. Nobody will get any spotting damage, neither you nor your ally.
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Note that the "eye" icon is irrelevant in all of this, it is simply an indicator of whether you are the first to spot (or re-spot) a target. If an ally starts spotting the same target as you do, nobody will get any spotting damage. Similarly, the assist stat is also irrelevant because it has nothing to do with assist damage.
From understanding these mechanics, assist damage can be farmed using several methods: a) making sure you're the only one spotting targets; b) de-tracking everything that your allies are or will be shooting at; c) designating everything your allies are or will be shooting at.
If you want to be extra cheeky, popping smoke in front of your allies (but behind you so you don't get blinded) will also allow you to rack up spotting damage as long as the smoke lasts, since your allies can still see the target thanks to you but they won't be able to spot it themselves through the smoke.
Now go be a cheeky sod and pad your assist damage stat. 