Look at what strong players actually slot and a pattern appears before any rules do. These are rosters from the builds STO BETTER has published.
The Arbiter Fire at Will build uses two Technicians for cooldowns, a Fabrication Engineer for healing, a Missile Weapon Officer, a Phaser Relay Officer that adds critical severity when Fire at Will activates, and a Dal R'El for power.
The Doolittle torpedo boat runs two Projectile Weapons Officers, one for critical chance and severity and one for reload, plus a Warp Core Engineer, a Space Warfare Specialist, and a Gravimetric Scientist, all Rare or Very Rare.
The Chronos tank uses a Quartermaster in the Technician role, a Fabrication Engineer, two Energy Weapons Officers, and a Security Officer.
The Fleet Nova exotic build prefers Epic officers, including Superior Romulan Operatives for exotic damage, a Na'kuhl Warfare Specialist, and Projectile Weapons Officers.
Every build fills its slots to reinforce whatever the ship already does: cooldowns for an ability driven build, projectile officers for torpedoes, exotic officers for science. Rarity follows budget, and the Doolittle build shows that Rare and Very Rare officers are enough for a top result. The rest of this page explains the system behind those choices.
How active duty works
The duty officer system unlocks at level 11. You get five active duty slots for space and five for ground, and only the space ones matter for your ship in combat. Buying the extra slot from the Fleet Spire raises each to six. Every officer applies its effect on its own, and depending on the specialization only one to three officers of the same type can be active at once. An officer posted to active duty becomes bound to your character, so check the variant before you slot an expensive one.
Each officer has a specialization, which is its role, and a variant, which is the exact effect it gives in space. Two officers with the same specialization can do completely different things, and that is the key to picking well.
Specializations used in space
Technician. One variant reduces the recharge of your other bridge officer abilities each time you use Auxiliary Power to the Emergency Battery, and another reduces bridge officer ability recharge directly. This is the cooldown officer, and the cooldown guide explains why it matters.
Conn Officer. Variants reduce recharge on Tactical Team, and on Attack Pattern Beta, Delta, and Omega.
Energy Weapons Officer. Variants reduce recharge on beam or cannon special attacks, or give stacking critical chance or critical severity when you fire energy weapons.
Projectile Weapons Officer. Variants reduce torpedo recharge, add kinetic damage, or give stacking critical chance or severity when you fire projectiles.
Warp Core Engineer. A variant can temporarily improve your power or remove debuffs when you use an Emergency Power ability.
Deflector Officer. One variant reduces other bridge officer ability cooldowns after you use one.
Gravimetric Scientist. Variants add aftershock Gravity Wells or Tyken's Rifts, or reduce cooldowns on exotic damage abilities when you create anomalies.
Photonic Studies Scientist. Reduces the recharge of Photonic abilities.
Space Warfare Specialist. One variant gives a chance for your energy firing mode at rank I or II to trigger the rank II or III version instead.
Why a second copy helps less than you think
Most duty officer effects are chances, not guarantees. The standard formula for the chance of at least one activation is one minus the quantity one minus the per use chance, raised to the number of same officers times the number of uses.
Take a hypothetical 10% chance per trigger, since real values vary by officer. One officer with five uses gives 1 minus 0.9 to the fifth, about 41%. Two copies with five uses each is 1 minus 0.9 to the tenth, 65%. Three copies reach 79%. The second copy adds 24 points and the third only 14, so covering several different specializations usually beats stacking copies of one.
Getting and choosing officers
Officers come from assignments, from packs in the Zen Store, and from other players on the Exchange. You can also turn in five underperforming officers, plus a modest amount of dilithium, for a random officer of the next quality up. Officers belong to a faction, and the game does not warn you when you buy the wrong one, so check before you pay.
To choose, start from your build. An energy ship wants Technicians or Conn Officers for cooldowns and an Energy Weapons Officer. A torpedo boat wants Projectile Weapons Officers. An exotic ship wants Gravimetric and Photonic officers. Fill the remaining slots with whichever variant matches the ability you use most. The Build Planner has Space and Ground Duty Officers tabs so you can plan a full roster, and Torpedo Boat Fundamentals and Exotic Damage (Science) Builds Explained show how officers fit into those two plans.