Obligatory Disclaimer (Read me)

Introduction
The NA/EU version of Black Desert Online currently contains a total amount four Field Bosses and one World Boss. They have a more or less fixed spawn timer based on their last spawn time. Players who hunt those Bosses can get the currently best loot in the game but the system on how the loot is distributed stays mystery for a lot of people.
I took my time to figure out a possible and logical distribution system for the loot based on my personal experience with hunting those guys.
It is hard to count the actual number of Bosses I have killed myself in combination with those bosses people killed and provided me information about but most days I kill at least 8-10 bosses a day always getting loot. Combined with the people inside my guild and some friendly people I made contact ingame with I think this theory is based on more than 50 kills per day for the last couple of weeks.

World & Field Boss – tl;dr
There are 7 Tiers of Loot for Field Bosses and 14 Tiers of Loot for World Bosses. A Field Boss will drop loot for up to 40 players while a World Boss drops loot for up to 80 players. Red Nose, Dim Tree Spirit, Dastard Bheg and Giant Mudster are Field Bosses while Kzarka is a World Boss. The more Boss Contribution (BC) you gather in the fight the higher your tier of loot. There are no guaranteed drops but the higher your tier the higher the chance of “good” loot – this means you can drop a piece of Boss Armor / Kzarka Weapon in the lowest tier or you get nothing in the highest tier. Your tier is shown by the amount of Seals you get at the end of the fight.
Everything you do at a Boss earns you Boss Contribution the more different sources you have the higher your tier (like a diminishing return for each source) – if you die you will lose some of your Boss Contribution. Your final tier is not a direct ranking against like if you 4th in Boss Contribution you are Tier X but the more people are at a world boss the less time you have to gather Boss Contribution due to the higher overall damage.
You will gain extra drops rolls for each first Hit you do and for the last Hit as well for gathering Contribution from each source.

Difference between Field & World Bosses (and Scroll Bosses)
First we need to clarify the difference between the two (three) types of Bosses ingame. Easiest to explain is the Scroll Boss – each day you get one T1 Scroll from your Black Spirit and each week you get one T2 Scroll. Those Bosses can be summoned using the Scroll at certain locations. The Boss that will spawn is based on the type of scroll you used. They mainly drop Black Stones and Hunter Seals (T1) for everybody in your party or even a Liverto Weapon (T2 Witch/Muskan) but they cannot drop parts of the Boss Armor Set. The Liverto from Witch/Muskan is also not a bundle but a fixed weapon. The Witch can drop a Witch Earring as well. In addition to the drops the person who summoned the Boss will receive 3-5 Reward Bundles based on the Tier of the Boss. Those Bundles can contain Boss Armor Parts and they are currently the only way to get the Muskan Shoes or the Giath Helmet.
Field Bosses spawn every 10-19 hours after their initial spawn. They spawn on all channels at the same time and all of them can drop a Liverto Weapon Bundle. In addition to the Liverto Weapon Bundle each Boss has his unique drops. Red Nose can drop his Armor, Bheg can drop his Gloves, the Dim Tree Spirit can drop a Tree Spirit Belt or his Armor and the Giant Mudster drops a Mark of Shadow. In addition those bosses drops Black Stones, Hunter Seals and various Gems. While your loot is personal if you do not loot it fast enough ~30-60s after the Boss dies it will become unprotected and anybody will be able to pick it up. A Field Boss drops loot for the Top40 people in Contribution. There are 7 Tiers of possible loot on a Field Boss.
Kzarka – currently the only World Boss – has a respawn timer between 25-35 hours after he spawns. He will drop loot for up to 80 people in a total of 14 different tiers. Besides the Shrine Guardian Token Ring Kzarka is able to drop the Kzarka Weapon Bundle instead of the Liverto Weapon Bundle. Like a Field Boss Kzarka will spawn in all channels at the same time but he will get a Server Announcement a few moments before he actually spawns.

Please Note: The following Numbers are completely made up and will only be used to explain the system. The actual Numbers should be impossible to figure out.

Boss Contribution
Everything you do at a Field / World Boss will give you Boss Contribution (short BC). As example: Everytime you deal damage to the Boss you will gain BC. There are multiple sources you can gain BC from.

Sources of Boss Contribution:
Damage Dealt: for each Point of Damage dealt you gain BC
Damage Received: for each Point of Damage received you gain BC
Aggro: for each second of Aggro you gain BC (it is possible that the BC gain increases the longer you hold the Aggro)
First Hit: for each first Hit you gain BC (You can first hit multiple times if the Boss resets in the fight)
Last Hit: Only one Last Hit can be done
Boss Mechanics: e.g. Musket on Bheg
Dying to the Boss: You will lose BC if you die to a Boss
Range: The further away you are from the Boss the less BC you gain

Latest information have shown that there is a possible diminishing return on each source. This means the more BC you gather from a single source the more you have to do for the next point of BC.
Imagine that damage dealt will give you 1 BC per every 100 damage dealt each point in the beginning but after 100 BC gathered through damage you will need to deal 110 damage for the next point of BC.
This can be expanded on each source of BC.
Dying on the other side will result in a huge loss of BC – it is not certain if you will lose % of your BC or a fixed amount for each death but I can certainly tell that the Diminishing Return for each source will be reduced with each death resulting in the ability to get back to your old highest point in less time.

About Loot Tiers and extra Rolls
First of all – once you are one of the Top40/80 players you will get loot but what you get is actually subject to complete RNG. However you are able to reduce the RNG increasing your Loot Tier and gathering extra Rolls.
Let’s talk about Loot Tiers first. This is how the Loot Tier table for Field Bosses could possibly look like:

Tier 7: >100.000 BC
Tier 6: >80.000 BC
Tier 5: >50.000 BC
Tier 4: >30.000 BC
Tier 3: >20.000 BC
Tier 2: >10.000 BC
Tier 1: >1 BC

The Loot Tier in addition to your Node Level, your Luck, your “hidden” Luck and other various possible Loot Chance increases determine your actual drop chance. This means you can drop any item already in Tier 1 but chances are much higher on Tier 7 but that also means you can drop nothing reaching Tier 7. Your Tier can be identified by the amount of Hunter Seals your dropped.

Extra Rolls on the other hand will use the drop Chance from the tier your reached. To gather extra Rolls you need to do different things in a boss fight (the following numbers are just examples not the acutal numbers). Let me give you an example:

Reaching Tier 7 will give you 7 additional Rolls, dealing damage for 20.000 BC will give you another 3, damage received for 10.000 BC +2 rolls, died once +1 roll, first Hit +1 roll and the last hit +5 rolls. This would allow you to roll a total of 19 times for loot with your drop chance from Tier 7.
However the amount of Loot you can get from a Boss is capped (possibly based on your Tier) so what the game basically does is once the Boss is dead roll X times and will give you the ?5? best Items diced. Yet thanks to RNG it is possible to simply roll 19 times hot air so even with a T7 and all those extra Rolls you will get nothing! (Did a T12 Kzarka Kill and got nothing but the 12 Hunter Seals) Or you are T3 with +4 rolls and you get a Liverto+Bheg Gloves.

Official Sources:
Field Boss 12h-24h Respawn
Field Boss drop Loot for up to 40 people
Kzarka Respawn 32h-44h / “Conditions” = BC Sources
Spawn Timer Field / World Bosses reduced by ~20%
Sources of BC: Q19