Tiers

Left 4 Dead 2 has what is called a 'Director' which chooses when and how strongly the players should be swamped by hoards of zombies and Special Infected and some other things (spawning items). These judgement are based on the strength of the players progress and how many zombies they are killing in a certain amount of time. The Director is a chaos manager.

Tiers in The Alternated Difficulties mod attempts to introduce chaos in a less managed state. On each map load a tier is randomly selected to define the pacing of the map. Where as a director is tactically selecting when things happen, a tier sets the stage for the map and away you go. Its the contents or logic of a tier that define them.

Tiers contain timers that reroll a sum of convars and director options on the fly as time passes in a map. Each tiers timers will trigger using a randomizer with different ranges. Much like gears, one tier's timers will trigger more often than another tiers timers. These timers adjust things like zombie spawn distances, mob recharge rate, zombie flow and reaction times and distances of zombies, mob sizes and spawn intervals.

Some Tiers timers have tight ranges for like 60-80 seconds other Tiers will have looser ranges like 40-120 seconds. When the timer goes off numberous convars are rerolled then the timer will get a new time from its ranges before rerolling the convars again.

Effectively, tiers have timers for chaos management that follow a strictly set randomized range of times that are not AI related unlike the director. The Director still functions, its just that the vehicle it drives is less predictable which makes it less useful than it was.