Flexible Raid

Flexible Raid (also called Flex) is a raid system integrated into Raid Finder, Normal, and Heroic modes. It allows for raid content to automatically scale its difficulty depending on how many players are in the raid at the time the bosses are encountered—between 10 and 30 players. That means if a raid has 11, 14, or 23 other people available for a Raid, they'll all be able to participate, and the enemies will scale dynamically. This difficulty scaling will bottom out at 10 players, even if you have fewer than that in the raid.

Raids using the Flex system operate on a loot-based lockout, allowing players to fight bosses they had already killed (but without subsequent loot).

World bosses also use this system to scale their difficulty.

History
In Patch 5.4.0, Flexible Raid was added as a separate difficulty that sat between Raid Finder and Normal difficulty (now Heroic) that was available for premade groups of 10 to 25 players, including any number in between. It was entered via queueing through the Flexible Raid UI and was separated by wings, similar to Raid Finder. It required a minimum of 8 players to queue. This difficulty was only available for Siege of Orgrimmar.

In Patch 6.0.2, the Flexible Raid difficulty was renamed Normal and this flexible scaling system was applied to Raid Finder, Normal, and Heroic (old Normal) difficulties for Siege of Orgrimmar and all subsequent raids. Heroic was renamed Mythic and remains the only difficulty to not use this system—being statically tuned around 20 players.

Breakpoints
There are generally no set breakpoints for boss abilities. With a few exceptions, each person in the raid adds a 20% chance for the boss ability to target an extra person or have an extra effect.

Progression
In Normal and Heroic modes, the bosses available in the raid instance is determined by the raid leader's loot-based lockout (first unkilled boss). If the raid leader has already completed all the bosses in the raid, it will start at the first boss.