RNG

RNG is an abbreviation of Random Number Generator. It commonly refers to game mechanics which have an element of unpredictability, usually governed by some code-based Random Number Generation algorithm. RNG or Randomness is present in Insta-Monkey selections, Late Game Round Generation, and most importantly, tower attacks as part of the damage types family. Examples include Sharp Shooter's Crits and Distraction.

In the context of the Bloons TD series, an attack consisting of RNG has a random chance of doing one outcome over another. Within Bloons Wiki, when an attack is tagged with Bloons Wiki's symbol for RNG,, depicted as the Sharp Shooter upgrade in Bloons TD 6, the attack relies on RNG for its power and cannot be adequately controlled by the player in any way shape or form, as opposed to Erratic which can be amendable with proper positioning or upgrades that make the tower shoot more accurately (An example being crosspathing Rocket Storm with Focused Firing). In the example with Sharp Shooter, it randomly crits after making between 8-12 hits, but the rate at which it crits cannot change because it relies on it making a random set number of attacks rather than positioning. RNG also applies with Distraction in two ways, the first and most significant example being the percentage chance of the blowback effect and the second and less significant example being the blowback quantity.

However, even though it cannot be directly controlled by the player, the player can minimize its effect. Taking Sharp Shooter again as an example, it can be crosspathed with Very Quick Shots to increase its attack speed so it can reach 8-12 hits faster. Further buffs like Primary Training and Overclock can also increase the Crit Frequency even more. Upgrading to Crossbow Master, the ability to Crit increases dramatically since the number of hits required to Crit has been lowered to 4-8 hits, on top of the fact that it shoots 6.25 times per second. Adding a Primary Training or Overclock, and the RNG factor is mostly negligible. Most RNG is tied to attack speed and/or the number of Monkeys that have the upgrade because it is usually the main attack that is affected by the RNG. 1 Distraction Ninja might not distract a lot of bloons, but 20 Shinobi Tactics can almost distract at all times.

One could also argue that the Crits aren't even truly random at all. True RNG would mean that there would be some games in which Sharp Shooter (and by extension, Crossbow Master) wouldn't be able to Crit at all even if they shot all of the time because it just so happens that the player got unlucky enough to never Crit at all. The only "randomness" in Sharp Shooter (and Crossbow Master) is when they reached to their 8th shot (4th shot for Crossbow Master), the game decides when it should Crit. It's guaranteed by their 12th shot (8th Shot for Crossbow Master) that the Sharp Shooter is going to Crit and it is also guaranteed that Sharp Shooter won't Crit in their 1st-7th shots (1st-3rd shots, for Crossbow Master).

Heroes Striker Jones level 5-18 making him can 50% chance to pop black and zebra bloons also count as RNG.

Upgrades with RNG
The following is a list of upgrades that involve RNG in their attacks, not including upgrades whose attacks rely on positioning (e.g. projectile deviation, on-track projectile placement, ricocheting behavior) rather than the outcome of hitting:
 * Sharp Shooter and Crossbow Master (random crits)
 * Blowback sources (randomness of blowback distance)
 * Distraction (both blowback distance and chance of blowback)
 * Druid of the Storm tornadoes and Superstorm supertornadoes (blowback distance)
 * Downdraft (blowback distance)
 * Robo Monkey, Tech Terror, and The Anti-Bloon main attacks (random crits)
 * Necromancer: Unpopped Army and Prince of Darkness undead bloons (pierce amounts, leading to increased graveyard consumption for higher-pierce undead bloons)
 * Psi's Psionic Scream (chance of blowback instead of instakill)
 * Striker Jones (level 5-18) (chance of pop blacks, zebras and DDTs)
 * Ball Lightning without Cold Front MK (chance of ball freezing bloons)

Upgrades formerly with RNG
Some upgrades formerly had true RNG attached to them. Others, such as Wall of Fire, have lost RNG by removing Erratic behavior rather than a true RNG factor. This includes, but is not limited to:
 * BTD6 Supply Drop's and Elite Sniper's cash crate rewards
 * BTD6 Support Chinook's and Special Poperations' cash crate and life crate rewards

Trivia

 * According to Ninja Kiwi's 5th November 2021 blog, Ninja Kiwi states that certain in-game elements use "seeded formulaics" rather than pure RNG. One of their examples of tower attacks with "seeded formulaics" is Heart of Thunder. On the same blog, they claim that RNG is "bane of co-op [sic]".