When do I use this ability?

You use it before the game begins. You’ll put Force Field onto Private Trailbreaker, then shuffle the rest of your deck and draw 3 cards for your opening hand.

Does Force Field work against damage done by a character with Pierce?

Yes. Even if the attacker has Pierce 5 or more, Force Field will make sure the Upgraded character takes only 4 damage.

Let’s say the Upgraded character also has Heroism. If another character would take 5 or more attack damage, what happens?

The damage that would be done to the defender is instead done to the character with Heroism and Force Field. That damage is still attack damage, so 4 damage is done to the Upgraded character instead.

What checks first when you have a Force Field on Jazz in Alt Mode? Would Jazz's ability reduce his damage to 2, or would Force Field reduce to it 4 and then Jazz's ability would reduce to 2 if he flips white pips?

Jazz’s ability would keep him from ever taking more than 2 damage, so Force Field would not trigger.

If your character takes 10 damage while it has a Force Field and a Backup Bag, will the Backup Bag save the Force Field from being scrapped while also reducing the damage down to 4?

It is Force Field’s own ability that is causing it to be scrapped, so Backup Back will apply. The character will take 4 attack damage and lose the Backup Bag.

Okay, so you've stated in the past that your opponent's characters count as "cards" when referring to Motormaster rulings. I was wondering, with regards to Spare Parts (which scraps itself instead of another Upgrade attached to your character) and Force Field, would a character attack count as a card causing Force Field to discard? In other words, if your opponent attacked an upgraded character with both a Spare Parts and a Force Field with enough damage to trigger the Force Field's effect, would you scrap the Force Field or the Spare Parts?

You would scrap Force Field because its ability is what causes it to be scrapped, not your opponent’s attack.

Megatron - Decepticon Leader in bot mode has a total of Pierce 6 due to his ability and the upgrade equipped (Crushing Threads) and attacks with a total of 5 regular attack damage. Prowl has (Force Field) equipped and defends for a total of 2 because no blue pips were revealed. Force Field does not trigger because the attack damage did not exceed the 5 and higher damage limit. Is this right? Does he take 3 regular damage or 6 pierce damage?

Because Megatron has Pierce 6, he would do 5 attack damage (remember that attack damage can’t be higher than the character’s attack, even with pierce). This causes the Force Field to trigger, so instead, he’ll do 4 damage and the Force Field will be scrapped.

Force Field states "If the upgraded character would take 5 or more attack damage - Instead scrap this card and the upgraded character takes 4 attack damage." If we follow the exact text of this and we have 2 Force Fields on the same character. If the upgraded character would take 5 or more attack damage both Force Fields would trigger, then they would take 8 attack damage (4 from each Force Field) This doesn't seem like the intended effect of Force Field but following the exact wording, isn't this what should happen?

No. Force Field is a replacement effect, and only one of those applies at a given time. Once that happens, it reduces the initial attack damage to 4, which invalidates the second Force Field.

If Dreadwing has two Force Fields equipped, wouldn't they both be scrapped at the same time? If a character would take 5 or more attack damage, both Force Fields would be triggered. Then you resolve them one at a time, in the order of your choice. The first one would scrap and reduce damage taken to 4. The second would scrap but the damage was already reduced to 4. This follows the Dinobot Sludge + Data Pad ruling where both effects are triggered at attack, and even if Data Pad puts your hand equal to your opponent, you still resolve Sludge's ability to draw 2 cards. Correct?

No. Force Field is not a triggered ability. See explanation regarding replacement effects above.

Force Field states "if the upgraded character would take 5 or more attack damage - instead scrap this card and the upgraded character takes 4 attack damage." What happens when the target received 4 or fewer attack damage? Do they take that damage and keep Force Field equipped until 5 attack damage is received or does it still get scrapped even if you have only taken 3 attack damage?

Force Field will not be scrapped if you take 4 or fewer attack damage on one attack, and it does not remember previous attacks, even in the same turn.

Does Grimlock's bot mode’s extra damage count as attack damage? If it does, and assuming Grimlock has 5 or more extra damage, how much damage does it do in the following situations: A. Damage is assigned to a bit with Force Field equipped. B. Damage is assigned to Insecticon Skrapnel in bot mode. C. Any bot that is protected by Motormaster alt mode skill.

The extra damage is not attack damage, so if you are redirecting to a bot-mode Skrapnel or a character with a force field on it, they will take the full additional damage. On the other hand, a damage-capping effect on the character you’re attacking will be relevant. So, if you have a Skrapnel in bot-mode with 5 damage on him and Grimlock attacks him for say 10 damage, that damage will be reduced to 3 by Skrapnel’s ability, 2 of it will KO him, and the last point will be assigned to another character of the defending player’s choice.

Skywarp in bot mode defends and I flip a white card and choose to redirect the damage to another character with Force Field. If the redirected damage is more than 5, does the other character's Force Field come in to play and reduce it to 4? Or is the redirected damage not considered attack damage and the other character takes the full 5+ damage?

Still attack damage, the Force Field will apply.

The opponent had an Optimus Prime - Battlefield Legend with an Anticipation Engine and attacked my Ramhorn. I had a Force Field in hand and was ready to play my white armor, when he flipped a Security Checkpoint with Anticipation Engine. Do I still get to play the Force Field in this situation or does the Checkpoint get rid of the Force Field before I play it?

Because attack triggers will resolve before defense triggers, in this scenario Security Checkpoint will get rid of your Force Field before you can play it.

On the topic of being able to choose the order of effects, how would/could Omega Supreme's bot mode ability work with another character having a Force Field?

If two effects would apply to the same attack damage, the player receiving the damage chooses one to apply. Then, if the other still applies, apply that effect. If no other effects apply to the attack damage, the attack damage is finished. For example, if you have a character with a Force Field and Omega Supreme, and that character would take 6 attack damage, you could choose either of the following: 1. Apply Force Field first, scrapping it and reducing the attack damage to 4, then use Omega Supreme’s ability to change the attack damage to: 2 to the other character and 2 to Omega Supreme. 2. Apply Omega Supreme’s ability first, changing the attack damage to 4 to the other character and 2 to Omega Supreme. Force Field no longer applies and will stay on the other character.