Dust is a unique thing within the combat system and plays by different rules. Each type of dust incorporated does one effect, as detailed below. The type of dust available for your character to use must be noted on the first post of any thread that involves combat, or you will not be allowed to use it in that thread. Dust abilities are as follows:
Steam Dust is a combination of Water and Fire, and creates a wet fog that obscures vision in an area roughly five meters in diameter for one post. Think of it like a low duration smoke bomb of sorts that focuses on getting ranged opponents off your back. If a character activates Steam Dust, any enemy trying to attack them from outside of the fog has their accuracy lowered by one rank for that post whether they're using their semblance, weapon, or any other ranged attack to account for the fact that the person who uses the dust is obscured. If the enemy is inside the area of effect, however, it does nothing because the fog is not very effective in close quarters. This is intended to do nothing to melee attackers, but be very useful against ranged attackers.
Earth Dust is a rarer type of dust than the primary four, and is not a combination of any of them. When Earth Dust is used, the very ground in up to an approximately five meter diameter of effect shifts and forces them to constantly make adjustments to not fall over and mess with their balance for one post. Think of it like putting on drunk goggles and trying to fight, it doesn't remove their ability to do it… it just makes it a heck of a lot harder. Any enemy trying to engage the user of Earth Dust within this area of effect has their accuracy lowered by one rank whether they're using their semblance, weapon, or any other attack. This is designed to be very useful against melee attackers, but useless against ranged attackers.
Water Dust is a type of dust that is often used by firefighters and other emergency management personnel and in day to day life. Its energy converts the dust into clear, drinkable water. It's not the most efficient way to create it, mind you, but when a fire's raging or someone has a handy electricity or water-based semblance… it can be pretty useful.
Ice Dust is the combination of Water and Air dust, and lets out a supernaturally cold burst of air at a target up to approximately five meters away. This cold burst of air lowers the speed of anyone hit with it by one rank for a duration of one turn as the area effected gains a very thin layer of ice that while easily broken does crucially slow down anyone caught within the area of effect.
Air Dust is one of the primary types of Dust, and causes a rush of wind to propel the user forward for a duration of one post. During that post, the user's speed is increased by one rank.
Lightning and Fire are two primary types of Dust that do the same thing mechanically. They inflict a damage over time.
Gravity Dust isn't explained very well in the source material, but basically just creates force that the wielder can use to propel themselves in any given direction.
As a result of the Argus Limited event, Atlas has provided the first in a brand of Limited Dust which is only available in specific events/scenarios. It is by default not enabled in any thread and in any thread where it is, there will be a note specifying that the Limited Dust types are available. This is done because these types of dust are in low supply and will only be used when required due to their high cost and rarity relative to other types.
Aura is a manifestation of the soul, a life force that runs through every living creature on Remnant, whether they are a meager shopkeeper or a renowned knight. However, what sets true warriors apart from all others is their ability to amplify and control their Aura.
Aura is primarily used as a defensive mechanism. Passively coating the wielder in a protective force field, it can protect a combatant from what would normally be a fatal blow. It does not, however, make the user invincible. As they receive more and more damage, their Aura reserve will deplete. If this happens, all the fighter will be left with is his resolve. Fortunately, when a fight turns gruesome, a warrior can also rely on their Aura in a different manner.
"Semblance" is a term used to describe the projection of Aura into a more tangible form. For some this could be the ability to control objects with telekinesis. For others it could mean superhuman strength. The power associated with a wielder's Semblance is completely unique. With enough training and focus, a user's Aura can turn them into something much more than just a man.
So, what does this mean in terms of mechanics for TRP?
It means that your Semblance, your Durability, and any combat abilities that rely on your semblance to work are tied with your aura. Once your aura reaches zero, you no longer have access to either ability unless your character has a mutation semblance or have time left in their transformation.
So how does your aura 'reach zero'?
Your aura depletes enough to 'reach zero' or be disabled typically in two circumstances: Gross semblance overuse and physical damage. The first category goes beyond just normal semblance use, and represents extremes like an ice creation and manipulation user making a wall that's 25 meters tall and 5 meters thick. This is way outside the norm, can only be maintained for a very short time, and would completely deplete the aura of anyone in seconds. Physical damage is much more straightforward, and the much more common way that aura can be depleted. If a character gets beat up enough, their aura will deplete. The physical damage part does not always mean combat, however, and aura can keep a person alive in a snowstorm for a time as well by shielding them from frostbite and the worst parts of the cold. This, too, depletes aura much like an attack would due to the aura preventing damage to the physical body of the wielder.
When an aura is close to breaking, it flickers. Every character on the site has a color attached to their aura, and when it hits critical levels there is a flicker of that color across the character's body. Semblance can still be used, and durability is still there… but the breaking point is very close.
When an aura 'reaches zero' that visible aura shatters. Whatever color represents the character's aura visually breaks and falls off of their body before dissipating into nothing. There is usually some sort of audio cue when the shattering happens too like glass breaking or the hiss of a shield dissipating. What's important here is once the aura 'reaches zero' it is blatantly obvious to everyone involved. No one would ever continue a fight with someone whose aura was shattered unless they wanted to seriously injure or kill them.
Yeah but bullets use Dust in Remnant, right? How does that work?
Dust applications aside from the ones listed in the tables above cannot grant mechanical benefits in Player Vs. Player Combat unless there is a combat ability that the character has that states otherwise. This is done for balance purposes only. If you want to start a fire with some fire dust or walk into a fire with some water dust in a noncombat thread nobody is stopping you. You just can't get any benefit from the stuff aside from the ones listed when a fight actually starts.
How is Dust usage different in PvE and PvP?
Simple: In PvP there are limits to the mechanical usage of Dust to ensure that it doesn't become a balancing nightmare because a player on TRP has the expectation of a level playing field where people can't just pull a giant wad of chain dust effects to overpower another person's semblance.
Grimm, however, get absolutely none of this. In PvE threads where it is relevant, there will be a note in the first post for the amount of Dust uses each character can have for the thread. This number will vary from thread to thread, but will usually fall between 2-4 uses for standard-length threads. This can take the form of bullets, other ammunition, or just a backpack full of crystals if that's what you want. The effects are the same, you just get a hell of a lot more of it. Overpowering Grimm with a bunch of dust is not only allowed, but thoroughly encouraged should you find yourself backed into a corner. Remember, kids, there's no kill like overkill.
In PvP the general rule is 1 usage of dust per thread, but this can be lifted if both participants agree. For tournaments and stuff like that, though, generally the number will stay at 1.
How do Aura and Health interact?
See the clarity update on this subject. The TL;DR is that aura and health are different pools, so damage taken to one does not effect the other. You can choose to take an attack with either aura or health, but not partially aura and partially health to prevent powergaming edge cases.