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I know this may be a stupid question but I have searched and not found a good answer. Can deffkoptas capture objectives or do they count as a vehicle since they are classified as jetbikes?
No, they can't claim objectives, but it has nothing to do with being a bike. They can't claim objectives because only troop choices can claim objectives, and they're fast attack.
If they were a troop choice and a bike or jetbike (like nob bikers when you take a warboss), then YES, they would be able to claim objectives.
The only things that count as vehicles are units with armor values. If they have wounds, then they're either infantry, bikes, jetbikes, or monstrous creatures, which can all claim objectives (as long as they're troops).
Now, if it's an apocalypse or planetstrike game, then anything can claim objectives, so they can.
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