Skill Identifiers (SI) are unlockable bonuses that can be chosen at the start of every game.  They grant certain effects that allow players to further specialize their Military Occupational Specialties into specific roles.  A maximum of two SI can be chosen, and they cannot be changed over the course of that game.

Most SI can be unlocked by reaching a certain amount of experience under the three Rank Sets - Enlisted, Warrant Officer, or Commissioned Officer.  Some SI, however, can only be unlocked by completing the game in a certain way.  Unlocking an SI under one of the Rank Sets makes it available to the other two, however not all SI are available for all three (e.g. Power User can be used as a Warrant Officer, but not as an Enlisted.).

Skill Identifier Abbrev. Enlisted XP Requirement WO XP Requirement CO XP Requirement Effect
Reactive Fire RF 600 0 (no XP requirement) 0 (no XP requirement) Next attack deals double damage every 8 seconds
Tough as Nails TN 2,000 0 0 Increases maximum life by 40
Quick Thinking QT 3,100 0 0 Reduces reload and unjam time by 25%.
Energetic EN 4,200 0 0 Increases energy regeneration by 0.5 per second
Hawkeye HE 8,000  1,500 1,800 Increases attack range by 2 and detects invisible units within 15 units radius
Intellectual IT N/A 4,000 0 Increases Mechanical Skills by 3
Always Ready AR 13,500 5,000 N/A Gives +240 experience and adds Quick Clot (2), Ground Flare (3), Smoke Grenade (3), and Ammo Case (4) to inventory. Stacks with Discipline [DS]
Whiz Kid WK 15,000 6,000 14,000 Increases explosive damage by 30% and adds 10 shields. Does not affect damage from indirect fire abilities (Orbital Strike, Call for Fire, White Phosphorus, Rail Gun, Call for CAS, and LLDR)
Twinkle Toes TT 20,000 15,000 N/A Increases move speed by 0.06
Power User PU N/A 11,000 N/A Increases energy restored by Power Cell, Combat Engineer's Arc Cell and Prototype's battery by 50%.  Adds Stacked Power Cell (5), Stacked Power Cell (5) to the inventory
Born to Lead BL N/A N/A 15,000 Grants an aura that increases base damage for friendly units by 5% in a 12 unit radius
Barbaric BA 25,000 12,000 19,000 Unlocks Rifle Butt (dose not work with cyborg and prototype)
Self-Sufficient SS 35,000 35,000 N/A Perform Self-Aid instead heals a random amount of life between 35 and 70 as well as removes minor and moderate ailments
Discipline DS 38,000 32,000 35,000 Increases experience gain by 12%. Stacks with Always Ready [AR]
Heavy Armor HA 50,000 20,000 25,000 Increases maximum life by 30% and reduces move speed by 0.1
Instructor IN N/A N/A 45,000 Grants an aura that increases attack speed by 6%, increases move speed by 0.03, and reduces reload and unjam time by 20% for nearby friendly units in a 12 unit radius
Combat Load CL 50,000 45,000 45,000 Increases the maximum number of magazines readied before Encumbered from 15 to 24

Increases the max amount of fuel readied before Encumbered from 1500 to 2400

Light Armor LA 50,000 50,000 50,000 Increases move speed by 0.10 and decreases maximum life by 25%
Field Support FS 80,000 80,000 80,000 Improves the Rifleman's Frag Grenade, Smoke Grenade, and Ground Flare abilities (cool down and energy cost)
Veteran VT 180,000 120,000 130,000 Increases base damage by 30%, increases move speed by 0.05, and decreases maximum life by 55%
Blood In Blood Out BO Complete Chapter 3 on Nightmare with Combat Medic. Improves the Combat Medic's Blood Transfusion and Nano-Injection abilities.
Steel Saber ST Complete Chapter 3 on Nightmare with Cavalry Scout Increases Cavalry Scout's base damage and range
Not Magic But Science Complete Chapter 3 on Nightmare with Combat Engineer
Battle Buddy BB Complete Chapter 4 on Nightmare (detain Dr Jashan) Start the game with a random Reinforcement unit
Lonely Soldier LS Win the game on Hard T1
Strong Together ST Win the game on Insane OB

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