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Maybe I'm wrong, but I think the guys in my league abuse the "true" line of sight definition. If I put a guy directly behind a unit, they say they can shoot THROUGH the unit to hit him because if they get a model's eye view they can see him through the unit. That just seems wrong to me considering people don't pose on the battlefield, they're constantly moving. The question is: can you shoot straight through units to hit a target behind them if you can see the target through the unit?
Yes you can.
If you can see an part of a models head legs arms or torso they have Line of Sight. Regardless of things in between.
To note though that if more the 50% of a model is obscured in such away, like if you can only see its head, or an arm, then they have a -2 to shoot from hard cover.
Pg 10 and 41 give the best information on this in the BRB