JJ typing, as always giving credit where credit is due. Thanks to Hulksmash for taking a few moments at FOYCON last year to confirm this information. We were saying the same thing just using different words, its a problem of mine.
Let's start with the rule in question from last Thurs game.
Area Terrain 40k6 pg 91
Area terrain is always difficult terrain. Models in area terrain receive a 5+ cover save, regardless of whether or not they are 25% obscured. Models that Go to Ground in area terrain receive +2 to their cover save, rather than +1.
Now for other cover save details:
Determining Cover Saves 40k6 pg 18
Area Terrain 40k6 pg 91
Area terrain is always difficult terrain. Models in area terrain receive a 5+ cover save, regardless of whether or not they are 25% obscured. Models that Go to Ground in area terrain receive +2 to their cover save, rather than +1.
Now for other cover save details:
Determining Cover Saves 40k6 pg 18
If, when you come to allocate a Wound, the target model's body (as defined on page 8) is at least 25% obscured from the point of view of at least one firer, Wounds allocated to that model receive a cover save.
Types of Cover Saves 40k6 pg 18
The type of cover save a model receives depends on exactly what he is sheltering behind. For example, a soft obstacle (like a bloodthorn hedge) that would hide soldiers behind it, but it would not even slow down enemy shots, confers a 6+ cover save. Purpose-built fortifications confer a 3+ cover save and most other things confer a 4+ or 5+ cover save.
Vehicles and Cover - Obscured Targets, second bullet point 40k6 pg 75
Vehicles are not obscured simply for being inside area terrain. The 25% rule given above takes precedence.
Context: This is a rule Shawn wanted me to put up when I looked it up after our game. I ran this rule incorrectly. Should've taken the time to look it up during the game. Psyflemen was in area terrain and was being shot at by Shining Spear Exarch. I was under the mistaken impression that Psyflemen gets a 5+ cover save because it was in area terrain. Shawn questioned it. Should've listened to him and looked it up. Because Psyflemen is a vehicle I shouldn't have taken 5+ cover save. grrrr, memory you've bested me again!
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