OPT-IN PvP flagging - No Corruption

You can. Just hit a dude. No additional benefits for that though, cause fuck that.

Nothing at all happens in that case.

If you fight back at all, you're considered "willing to PvP" and anyone can kill you. (Otherwise if your original attacker had a friend with them, the friend might get corruption.)

If a mob kills a player, nobody gets corruption. Steven's chosen solution to the "cheese" of letting a mob last-hit is to make it so you don't know how much hp other players have left and might accidentally kill them yourself. I'm not sure how this helps with "making their even-match PvE suddenly become difficult due to low health though. You can still tell if a player is high or low generally.

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Sure as hell hope this will not be the case. I plan to help out any of my citizens and if they get attacked and I can only literally stand there and do nothing because I'm 10 lvls higher than their attacker - I'd be very disappointed in the game.

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Exactly, so it doesn't need any of that "high lvls deserve corruption if they attack lower leveled flagged people" stuff

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It's that purples turn green after not fighting anyone for 90 seconds, so purples aren't just "in the world". To be purple means you just hit another player. Maybe to attack, maybe to defend.

This probably covers what the OP requested, but the OP might be asking for something more like "please mark me as always purple", so that players could identify themselves easily as "open to PvP" rather than everyone looking like a sea of green, with no hint as to whether they're actually interested in dueling.

(It could also be mildly confusing to someone new to the forums/wiki who didn't fully understand the system yet, which is another possibility. I'm sure OP can clarify.)