It seems odd to have a rules question since 5th edition has been going for so long but my local gaming group is in the early parts of a large campaign and we have all decided to avoid disputes by eliminating our house rules and going "by the book" on everything. This new resurgance of studying the rules has opened a storm in the area of true line of sight involving vehicles.
One side believes that if a unit is shooting through a patch of woods or a unit of infantry, that the trees/models in that woods must cover 50% of the model completely for the tank to recieve the cover save.
The other side believes that the area terrain description shows that if the unit is firing between the models/trees then that cover save still applies even if the tank is completely visible to the eye.
I personally side with the second interpretation but I would like to know what everyone else thinks. Has this problem come up in your groups? How did you resolve it? also if you can point to specific referance pages in the rule book that would help to make the case.
without checking the rules right now, I'd go with the at least 50% obscured side.
The rulebook specifically mentions true line of sight doesn't apply to area terrain. If something like a wooded area has a delineated border (i.e. a base) that is what you use to determine if a target is in cover. Modeling dense woodland is sometimes impractical and so the border is used instead.
The 50% rule holds true if the vehicle is sticking halfway out from behind area terrain, though.