Additional game terms that are most often relevant during combat.

Cover

Derelict vehicles, trees, chest-high walls, and other obstacles can provide cover during combat, making a target more difficult to harm. A target can benefit from cover only when an attack or other effect originates on the opposite side of the cover. 

There are four degrees of cover which are, in ascending order: partial, half, three-quarters, and full. If a target is behind multiple sources of cover, only the highest degree of cover applies; the degrees aren’t added together. 

A target with partial cover has a +1 bonus to AC and Dexterity saving throws. A target has partial cover if an obstacle covers a portion of it, but not quite half of it. This obstacle might be a moderate piece of furniture, or rubble that doesn’t cover a notable portion of the target.

A target with half cover has a +2 bonus to AC and Dexterity saving throws. A target has half cover if an obstacle blocks at least half of its body. The obstacle might be a low wall, a large piece of furniture, or a narrow tree trunk. 

A target with three-quarters cover has a +5 bonus to AC and Dexterity saving throws. A target has three-quarters cover if about three quarters of it is covered by an obstacle. The obstacle might be an armored vehicle, a bunker slit, or a thick tree trunk. 

A target with full cover can’t be targeted directly by an attack or ability, although some abilities can reach such a target by including it in an area of effect. A target has full cover if it is completely concealed by an obstacle.

Concentration

Some features, spells, Powers, and other sources require you to maintain concentration in order to keep their effects active. If you lose concentration, the duration of the source ends.

If a source must be maintained with concentration, the fact that it does so will be mentioned, along with the maximum duration you can concentrate on it. 

Normal activity, such as moving and attacking, doesn’t interfere with concentration. The following factors can break concentration:

• Invoking another source that requires concentration. You can’t concentrate on two sources at once.
• Taking health point damage. Whenever you take health damage while concentrating, you must make a Constitution saving throw to maintain your concentration. The DC equals 10 or half the damage you take, whichever number is higher. If you take health damage from multiple sources, you make a separate saving throw for each source of damage.
• Being Incapacitated or being reduced to 0 hit points. You lose concentration if you are Incapacitated, or reduced to 0 hit points.
• Choosing to end concentration. You may end concentration at any time (no action required).

The Architect might also decide that certain environmental phenomena, such as a wave crashing over you or an artillery cannon firing right next to you, require you to succeed on a DC 10 Constitution saving throw to maintain concentration.

Visibility and Hiding

There are four stages of visibility, described next from most visible (highest) to least visible (lowest). Your stage of visibility affects how you and other creatures interact with each other. It’s also determined on a case-by-case basis. While moving behind a wall might make you Unseen to some, any creature on the other side of the wall with you might still see you as Clearly Visible.

Notably, your stage of visibility doesn’t affect how you make saving throws, not even against Payload weapon attacks. You’ll need some form of cover to gain a benefit to your Payload saving throws.

Hiding

Being hard to see, even so far as to be Unseen, doesn’t automatically make you Undetected to others. Creatures might still know you’re in the area or have some other means of detecting your presence.

Regardless of your stage of visibility, you need to take the Hide action if you want to become Undetected. Make a Stealth check, the result of which becomes your Stealth DC. Use this result even if it’s lower than a previous Stealth DC of yours. 

You are considered Undetected to creatures whose passive Perception score doesn’t meet or exceed your Stealth DC. Alternatively, a creature can use its action to make a Perception check. If this beats your Stealth DC, you aren’t considered Undetected to them until you take the Hide action again.

In most circumstances, being Undetected to a creature means it’s unaware of your presence entirely. An unaware creature won’t even know to make Perception checks to find you, though if your Stealth DC isn’t high enough and you aren’t Unseen, passive Perception can still be used to detect you. 

But being Undetected doesn’t inherently mean other creatures are unaware of your presence. For instance, if one creature beats your Stealth DC, they might then call out to their allies, “Hey, there’s a Guardian here!” Until their allies also beat your Stealth DC you are still Undetected to them (being aware that a creature is nearby isn’t the same as being able to detect the creature), but now they know to begin looking for you.

Like with visibility and cover, whether or not you’re Undetected is determined individually from one creature to the next. Also, taking the Hide action doesn’t itself change your stage of visibility. 

Detecting the Undetected

Your stage of visibility determines how a creature is able to detect you when you’re Undetected. A creature’s extraordinary senses might further alter how they’re able to detect you.

Even if you lose the Undetected condition, your stage of visibility still affects you.

• Clearly Visible & Undetected: You are immediately detected if a creature is able to clearly perceive you with any sense, even basic sight.
• Lightly Obscured & Undetected: Creatures make Perception checks to detect you normally.
• Heavily Obscured & Undetected: Creatures have disadvantage on Perception checks to detect you. 
• Unseen & Undetected: Creatures can’t detect you with sight alone, even if their Perception check or passive Perception score would exceed your Stealth DC.

Die Size Changes

Some features, weapon perks, or other sources may instruct you to increase or decrease the size of one or more dice. For instance, the Versatile weapon property says to increase the weapon’s damage die size by one when the weapon is used with two hands to make an attack. If the weapon’s damage is normally 1d8, that 1d8 would become 1d10 when using the weapon with two hands to make an attack. If the weapon’s damage is instead normally 2d6, it would become 2d8.

Increase Die Size. The value of the die or dice goes to the next size up. A d4 becomes a d6, a d6 becomes a d8, a d8 becomes a d10, and a d10 becomes a d12. You cannot increase any die above a d12 unless the source explicitly allows you to.

Decrease a Die Size. The value of the die or dice goes down. A d12 becomes a d10, a d10 becomes a d8, a d8 becomes a d6, and a d6 becomes a d4. Die sizes can never be decreased below d4.

Line-of-Contact

If you could hypothetically run a straight length of string between one point and another, there is line-of-contact between the two points. Line-of-contact does not require visual sight. Walls, doors, and other tangible objects break line-of-contact, but smoke would only break line-of-sight and not line-of-contact. Conversely, a window breaks line-of-contact but not line-of-sight.

If a trait, feature, Power, or other source requires line-of-sight (“a spot you can see” or “a target within sight,” for example), it is inherently assumed the source also requires line-of-contact. A source that only requires one and not the other will mention as much in its description.

Projectiles

Projectile refers to a ranged solid object or tangible mass of energy that is not also described as a beam, wave, or shockwave. For example, an arrow shot from a combat bow or a nova bomb cast by a voidwalker are both projectiles. A melee attack from a dagger is not a projectile, but if you threw the dagger, it would then be considered a projectile. A shot from a trace rifle is not a projectile, because trace rifles have the Energy Weapon property.