Unwritten Rules

"The code has been there since the beginning. If a bigger threat shows up, we band together. We don’t distract each other with attacks or murder attempts, we don’t take advantage of the situation to fuck with civilians. The truce is there for a reason, and it has weight because everyone knows that they can’t handle the trouble that gets express-delivered to their doorsteps when they’ve defied it."

- Taylor

The Unwritten Rules, also known as the code, the truce or the game, were a semi-unofficial code of honor adopted by capes. It existed in multiple countries, including much of North America and Australia, but not Russia, which had its own code, or the CUI.

Known Rules

 * Respect the secret identities of fellow parahumans. Private lives are a needed outlet and taking that away is asking for trouble.
 * No attacking the civilian family members of parahumans.
 * No violence or using powers during a meeting between multiple parties.
 * Try not to use lethal force.
 * No widespread attacks against civilians.
 * Enslaving others with mind control is not permitted.
 * No rape or sexual assault of any kind.
 * Don't use guns. If you absolutely have to carry a gun be careful using them.As stated by others, it depends on what you do, not what you have. A lot of people (especially backseat critics who like to go on at length about what in-story characters are doing wrong) tend to say 'Gawd! just bring a gun, shoot them!', but in reality carrying a gun tends to say a lot, changing all interactions surrounding a cape. Knives and other weapons do to, but carrying something like that can often be considered almost an extension of the costume/image. Guns fit into that less. Carrying a gun adjusts all interactions. You're saying 'I'm not in this to play' or 'I'm expecting to hurt seriously and risk being hurt, and I'm prepared to defend myself'. It escalates the game and makes everyone a little more tense. A gang of thugs that all carry guns are going to come across as much more serious and will be treated as such. Getting arrested while having a gun, even if you don't use it, the group is going to get hit that much harder. Less so for a single individual carrying a gun in a larger group. In short, it's discouraged. Keep in mind that I'm talking about handguns here. Rifles, SMGs, shotguns, the context is much different. You don't bring a SMG to perform more effective warning shots. Using a gun, though, that changes things. The moment someone gets shot, it stops being theatrics and posturing and becomes an omnipresent danger. The game is no longer being entertained. In large part, context aside, the shooter is giving up the unwritten rules that would benefit them. Tattletale played by the rules. She shot Glory Girl to disable the forcefield. Still softballing it. Skitter shot Coil, but there was a longstanding relationship (and it was fairly discreet). Still sketchy, still a problem. Doing it with no warning, shooting a civilian, doing it recklessly enough civilians get caught in the crossfire, doing it against someone weaker, all will be perceived as dangerous and reckless. Maybe the entire city's cape population doesn't band together to go after you like the ABB did, but others may start playing it like the referee isn't watching anymore. You're the guy who shot Smokey Bandit, I don't know the whole story, and I'm not going to give you the benefit of a doubt and fight you one on one. I'm going to blindside you with two of my buddies at my back. Oh, you want to join Dark Lord Prowess' team? Well he doesn't want a jackoff who shoots B-listers working for him. Hm, you want to deal with me? Work together? Fuck you, it's your recklessness that's scaring civilians and making it harder for me to keep stuff running smoothly. The way the rules work, people know what's up and generally things work against those who pack heat and those who use it recklessly. They end up alone, they end up with more enemies rallied against them, hitting harder, and generally they get enough warnings that they should cut it out that they'll quit it or commit. Typically the approach of the opposition will be 'we know he uses guns, so let's not make this a thing that keeps escalating'. In different areas, though, the situation may well be different. I can picture Texas having more gun-wielding capes in general. - Wildbow on Reddit They lead to escalation and death.
 * Stick to less "lethal" archaic weaponry, swords crossbows shields, things that work with powers.
 * A subset of rules are applied in areas currently under attack by Endbringers, and other S-Class threats.
 * No attacking people who volunteer, irrespective of grudges.
 * No taking advantage of an attack for personal gain or to advance your faction.
 * No setting up others to die.
 * Medical care is provided without discrimination for past deeds.
 * If anyone breaks the rules, all willing parahumans work together to effect punitive action.
 * Repeated and flagrant breaking the rules could be grounds to be sent to the Birdcage or for receiving a kill order.

Limits
The rules varied somewhat from place to place.

It was somewhat debatable whether the rules applied to non-parahumans like the PRT.

Variants
The underworld in Russia doesn't let their Parahuman members fight each other, in order to conserve strength to fight outside forces.

Background
After parahumans first emerged, it took some time for the unwritten rules to evolve. Gavel for example, was initially tolerated for attacking the families of supervillains.

Story Start
The ABB were fought together by both heroes and villains after they began enslaving civilians and bombing the city.

Releasing the identities of the Empire Eighty-Eight was a violation of the unwritten rules.

Post-Leviathan
Skitter was accused of breaking the unwritten rules by allegedly taking advantage of the Battle against Leviathan to learn Shadow Stalker's secret identity. As a compromise, it was suggested that she could reveal her identity to Shadow Stalker, for which there was precedent.

Armsmaster was considered to have broken the unwritten rules by steering villains into the path of Leviathan without telling them.

With the arrival of the Slaughterhouse Nine the capes of Brockton Bay met to convene a strategy. Emily Piggot violated the truce by calling in an airstrike while heroes and villains were still battling the Nine, risking their lives.

Post-Slaughterhouse Nine
The Undersiders were repeatedly warned that making use of the Echidna situation against the Protectorate would be considered a violation of the truce, and likely result in a kill order being placed on them.

Assault and Clockblocker argued that the Undersiders were violating the unwritten rules, and the heroes should consider escalating in kind.

Post-Echidna
The Brockton Bay Protectorate under James Tagg violated the unwritten rules by unmasking Skitter, on the advice of Dinah Alcott. Technically, this succeeded.

Chevalier considered that "the truce was in worse shape than it had been even in the beginning". Perdition, a rogue member of the Yàngbǎn, violated the truce by attacking Chevalier, Accord and Tattletale during the New Delhi battle.

Weaver was accused of breaking the truce by cooperating with Phir Sē.

Gold Morning
The Elite were destroyed for raiding civilians for resources and pushing others out of their claimed territory during the Gold Morning.

Post-Gold Morning
Two years after the event a general amnesty for all capes, a "second chance" of sorts, became a part of the unwritten rules. It's implementation has had inconsistent success, With some people and groups becoming heroic and welcomed with varying levels of acceptance. While others were shunned and doubled down on their villainy.