The Old Code

The Old Code, also simply called 'The Customs' is a set of a widely accepted forms of courtesy and terms of engagement, establishing procedures for things such as hostage exchanges, negotiations, rites of passage, neutral territories, laws of hospitality, trade, neutrality and other such things.
The Code is known to most scholars and most organizations across the realms follow the terms outlined in it to one degree or another when interacting with each-other or with outsiders.

Those that invoke the Old Code gain both its obligations and its protections.
Breaking the Old Code once it is invoked is generally a bad idea. There is multiple groups throughout the realms that are actively backing the customs and that have a vested interest in maintaining the code.
This is not a magical affair, the agreement is perfectly mundane. Parties try and succeed in breaking the accords, and though it has negative consequences for the offending party if discovered, many still attempt it.
It is to everybody's benefit keep the accords intact, so while transgressors may not be fought openly by other groups, they will also not be helped (at least not publicly).

Established Procedures:

The Old Code establishes the following:

Duels

Established procedures for settling conflicts.
The accuser can challenge an accused to a duel, for any reason. If the accused refuses the duel, then it won't happen.
If both parties agree to duel, both must select a 'second', who will negotiate the terms of the duel and observe the results.
The conflict is overseen by a neutral party which is chosen by both the accuser and the accused (or their second).
The accuser selects the time and place, the accused selects the weapon/test/contest from a list of options. The neutral party decides what options are on the list the accused chooses from. If the accused chooses a weapon/contest the accuser cannot use, the accuser can force the accused to pick his second choice.

Neutral arbiters & territories

neutral Territories: The Accords establish procedures for neutral territories. Neutral territories are locations that are designated 'safe'. They are negotiation spaces for hostile groups to come to amicable agreements. Parties that invoke the Old Code are not permitted to do battle here. They are obligated to take any physical or magical confrontation outside of the territory.

Neutral Arbiters: Neutral parties are those that agree to arbitrate in case two parties wish to have a neutral party to cast judgement. How 'Neutral' these parties are can vary greatly. The most famous neutral party are perhaps the Mechanoids that arbitrate conflicts brought to them in Ezria.

Laws of Hospitality

Hosts: If somebody has promised safe passage under the Old Code, they are obligated to not harm their 'guest' and to protect them in case a foreign entity attempts to harm them. They are expected to provide food and lodging, if the guest requires it. All of this only applies if the guest also swears to be a proper guest.

Guests: If a person promises to behave themselves as a guest should under the Old Code, they are obligated to respect the host's customs. They are not to harm the host or any under their protection while they stay inside their domain. They are obligated to not abuse their host's generosity and not cause them undue trouble. They are obligated to leave if the host asks them to do so. All of this only applies if the Host promises them safe passage.

Signatories

The Old Code is Backed and supported by several parties that have a vested interest in its maintenance. these parties include:
The Divine Machine
The One Above All
The One Below All
The Fates
The Fairy Queens
And many more