If someone has "natural" body armor, can they gain additional protective benefit from wearing artificial armor on top of it or is this an either/or situation?

The Advanced Players book (pg. 29) states that with Force Fields, one must choose to use either their body armor or the field but not both (unless you project the field as a sort of shield to protect someone else, in which case both apply). I couldn't find any specific ruling regarding armor stacking, though.

Example: A typical Asgardian (who all possess Good protection from physical and energy attacks, Judges pg. 60) decides to visit our dimension and finds himself hooked up a squad of soldiers on a mission. They equip him with a Flak Jacket for protection, not knowing his innate abilities. Our Asgardian visitor is fired upon by a foe armed with a Machine Pistol. The Flak Jacket provides Good protection. The Asgardian's flesh provides Good protection. The Machine Pistol inflicts 20 points of damage on a hit. Assuming standard ammunition is used in the attack, does the Asgardian have to choose whether he will be protected by either the Flak Jacket or his innate body armor and soak up 10 points of damage, or does he gain the benefit of both and take no damage (though still subject to Slam/Stun/Kill results, if any)?

I realize this rule was probably written as proofing against a player character making himself invulnerable to attack by coating himself in layers of protection. Still - I'm curious.

You get the best value of one of the two.

I would also suggest that there might be additional factors involved other than choice. For example, let's say you have a breast plate that grants RM (30) protection and chain mail that provides GD (10) protection. Clearly, on its face value, the breast plate is the better armor. But what if an enemy gets a bullseye result and aims for one of the unprotected spots? The chain mail covers a lot more, so one might be better off with it if expecting enemies to be shooting from different positions whilst engaging another foe in melee.

I prefer to just let armor stack even with built-in defensive/protection powers, because it makes more sense to me that way. But I also don't let characters just slap on a whole lot of armor one on top of the other without some kind of drawback or penalty to flexibility or movement, because let's face it, irl if you just wear a whole lot of clothes all at once, you're going to bog yourself down.

Yes they do have a ridiculous rule to prevent even reasonable stacking of protection so that you've something as ridiculous as someone's natural defenses being bypassed by an attack because they also wore body armor. So for example if you accept the rule Super-Skrull can't stack his force field with his natural body armor yet Susan Storm can project her Force Field over the Thing and he'd get the benefit of both just fine.

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Nightmask
for example if you accept the rule Super-Skrull can't stack his force field with his natural body armor yet Susan Storm can project her Force Field over the Thing and he'd get the benefit of both just fine.

That technically does not happen. The way projected force fields work according to the rules as written, if damage exceeds a force field's rank the force field collapses but the additional damage is not transferred to those protected by the force field. Basically all damage is protected from for the attack that negated the force field... so even if Galactus hits with a CL1000 energy attack... Sue's force field soaks it and then goes away. The Thing's natural armor doesn't get added in to the protection because the damage doesn't get to him until any additional attacks after the force field collapses and until a new force field can be established.

Per the Advanced Player's Handbook under the Force Field power description:

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Advanced Player's Book, page 83
What occurs with a failing force field depends on its type. A personal force field shuts off, and the excess damage is taken by the user, including any effects from the battle effects table. The failure of a projected force field results in the force field coming down, but those inside are unharmed by that attack. The wielder of the force field must make a Psyche FEAT roll against the Intensity of the attack or become unconscious for 1-10 rounds.

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Maniacles
I prefer to just let armor stack even with built-in defensive/protection powers, because it makes more sense to me that way. But I also don't let characters just slap on a whole lot of armor one on top of the other without some kind of drawback or penalty to flexibility or movement, because let's face it, irl if you just wear a whole lot of clothes all at once, you're going to bog yourself down.

In my breast plate vs chain mail example above, I would absolutely allow the breast plate to be worn over the chain mail, but I wouldn't give greater protection for it. Rather, the character gets the overall better protection of the breast plate, but if someone were to use bullseye results to strike the weaker armored points, the character only gets the weaker protection of the chain mail.

I would hold the same true for layering armor over natural armor. You only get the benefit of the better armor. If the better armor is the worn stuff, then its protection applies in general but the character still has their natural protection for any attacks that bypass the worn armor. However, if the character's natural protection exceeds the worn armor, then they receive no benefit from the additional layer... at that point, it's just clothing --a fashion accessory that indicates they are a warrior who dresses for battle.

Once again, citing the force field power description as an example:

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Advanced Player's Book, page 83
A personal force field is considered to replace Body Armor, such that damage that gets around a personal force field is not absorbed by Body Armor. (You cannot layer Body Armor with a force field to become a tank, though the two together give you options.)

You don't get the benefit of both added together, but having both gives you options... as in being able to resist damage when one or the other fails (or is ignored) for whatever reason.

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Descartes Demon
If someone has "natural" body armor, can they gain additional protective benefit from wearing artificial armor on top of it or is this an either/or situation?

The Advanced Players book (pg. 29) states that with Force Fields, one must choose to use either their body armor or the field but not both (unless you project the field as a sort of shield to protect someone else, in which case both apply). I couldn't find any specific ruling regarding armor stacking, though.

Example: A typical Asgardian (who all possess Good protection from physical and energy attacks, Judges pg. 60) decides to visit our dimension and finds himself hooked up a squad of soldiers on a mission. They equip him with a Flak Jacket for protection, not knowing his innate abilities. Our Asgardian visitor is fired upon by a foe armed with a Machine Pistol. The Flak Jacket provides Good protection. The Asgardian's flesh provides Good protection. The Machine Pistol inflicts 20 points of damage on a hit. Assuming standard ammunition is used in the attack, does the Asgardian have to choose whether he will be protected by either the Flak Jacket or his innate body armor and soak up 10 points of damage, or does he gain the benefit of both and take no damage (though still subject to Slam/Stun/Kill results, if any)?

I realize this rule was probably written as proofing against a player character making himself invulnerable to attack by coating himself in layers of protection. Still - I'm curious.

stacking in marvel is easy: if they are within one rank of each other, add +1 CS.

your asgardian (good BA) wearing a flak jacket (good BA) would have excellent(16) body armor. he'd suffer 4 points from the 20 point attack, and would be vulnerable to stun/kill.

if you did have 0 damage, special results like kill generally do NOT have any effect.

Frankly, you might overrule the forcefield bit too, it doesn't make much sense. Mrevel's underlying rules are pretty simple & effective. stacking is rarely going to be a problem, as per the rules, it's only effective for things within a rank of each other. (sometimes 3 things even, good+good(=excellent)+good=remarkable, but all too often things wouldn't stack at all, such as good +remarkable=remarkable). shifting up always results in the lowest number inside the rank too (as shifting down results in the highest)

Combat is abstract, but I don't see an issue with stacking armor.

By the rules, no you can't, but if it came up I'd house rule it works just fine.

I look at it like this: If I put a flak jacket on a jello mold and one on a cement pillar and shoot each of them, the cement pillar is going to fair much better when I breach the flak jacket.