Every survivor is defined by their skills - a catch-all game term that can refer to a survivor's unique knowledge, background, birth traits, expertise and much more. All of them are considered skills and all of them appear in the survivor's character sheet together under Skills. Skills could offer a passive bonus to a survivor, enable them to perform some action at the base, equip some rare piece of equipment that they are unable to do so, or offers them an Active Ability they could use a limited amount of times during a battle.
While all skills are grouped together, there are actually three major types of Skills: Profession Skills, General/Negative Skills and Skill Trees. There are also two additional skill groups that are smaller and rarer: Learned Skills and Unique Skills. All survivors, with the exception of Unique Survivors, have skills in all three major types.
Finally, there is a category of Unlock Skills that can be obtained in various ways and means, but are key - as they unlock access to certain high-end weapons, armor, and objects.
The skill groups vary greatly in the potency of their skill and how a player may go about obtaining them.
Skill trees are assigned to a character based on their profession. As a character gains experience and levels up, they can increase their progression across skill trees by assigning ability points to their trees. Survivor gains one ability point per level, with one additional point at level 5 and 10 - with 10 being the maximum level. Therefore, a player can get up to 12 ability points.
Every profession offers a survivor 8 different skill trees to choose from. Every skill tree has four levels, with the fourth level usually unlocking a powerful capstone skill. All in all, a player can hope to maximize 3 out of the 8 skill trees of their survivor.
Some levels have multiple options, of which only one can be unlocked when an increase is selected. Some of these options are available across various skill trees - unlocking them once removes the ability to select them a second time from another skill tree.
Every survivor - with the exception of unique survivor - is generated with 1 profession skill at the start of the game. They can not be obtained or learned in any other way. These skills are the most powerful and potent skills in the game - they are meant to be the biggest differentiator of survivors, and what truly defines them.
Profession Skills reflect that survivor background - what they have done in the world before the Apocalypse. They are called Profession Skills because they are selected at random from a pool that is unique to every profession. A Fighter profession skill could be an ex-cop, with an Academic's profession skill might be a mathematician.
Every Profession has a pool of at least 8 different Profession Skills, ranging in three levels of rarity - common, uncommon and rare. Rarer profession skills tend to be overall better than those of lower rarity but beyond that, they have no meaningful difference.
Most survivors end with more than one starting skill. Like Profession Skills, general and negative skills cannot be learned or changed during the game (with the exception of Mutate Soul Ritual). Unlike Profession Skills, where a survivor always has one, survivors may have anywhere between 0 and 3 general and negative skills.
General Skills are mostly positive benefits. They are of a lower benefit than Profession Skills but could still be quite potent. Players may have 0, 1 or 2 general skills, with 33% chance for each of those options. General Skills also range, like Profession Skills, in rarity, with common, uncommon and rare options.
Negative skills are, as can be expected, mostly negative benefits. A survivor has 25% of having a negative skill. Negative skills do not have rarity levels.
Certain Professions may disallow certain skill combination (both negative and general skills). So, for example, one can not have a Skeptic Priest or a Hippie Priest.
Learned Skills refer to a short list of skills that can be obtained via rare books, that are sold by traders. They all, without exception, unlock new Active Abilities, and survivors can learn them by using the book they purchased from the trader.
Some of those active abilities, or version of those abilities, can be obtained via general skills, profession skills or skill trees. In those cases, the learned skill will stack with those skills, giving the survivor another 'charge' of the active ability.
A small assortment of survivors that exist in the game pool is called Unique Survivors - preset survivors that are meant to be a homage to a great hero from modern folklore, usually a TV, movie or novel reference. Those survivors can be recognized by their unique and highly powerful Unique Skill - no other survivor will have it.
Unique Skill basically replaces the typical Profession Skill most survivors have, and it far more powerful than any profession's skill. Since unique survivors do not start with any other negative or general skill, those unique skills are usually more powerful than any combination of rare general skills and rare profession skills.
Unique Survivors are a highly powerful prize and should be treasured and protected. That being said they tend to be somewhat of a cookie cutter - with highly specific and individual advantage that is usually good for one specific use.
Unlock Skills are an a-typical category that is used mostly to simply have an easy reference to a certain range of skills. These skills can usually be obtained as a bonus from various general and profession's skills, or they can be gained via skill trees. In the case of skill trees, they are almost always available as an elective in the capstone skill of that specific skill tree - ie, the final fourth level of that tree.
Without exception, unlock skills are major milestones and allow a survivor to equip or build the most powerful gear in the game.
Defensive Structures: This allows the survivor to construct powerful defensive structures and objects, such as the Guard Tower and Bunker. It can be learned by reaching the capstone in the Construction skill tree.
Sniper Rifles: This allows the survivor to equip the powerful yet unwieldy sniper rifles, including the Sniper Rifle, 50 Caliber, and Guided Sniper. It can be learned by reaching the capstone skill of the Marksmanship, Scouting or Rifles skill trees.
Rare Firearms: This allows the survivor to equip any range of rare ranged weapons, including Pistols, Automatic Weapons, and Rifles. It can be learned by reaching the capstone skill of the Marksmanship, Damage, Pistols, Rifles or Automatic Weapons skill trees.
Rare Tech Weapon: This allows the survivor to equip rare tech weapons, such as the Coilgun, Railgun, Freeze Ray, Beam Cannon, Damnator and Plasma Charger. It can be learned by reaching the capstone skill in the Research, Tech Weapons, Automatic Weapons or Rifles skill trees.