Fleet Commanders are a separate layer of officer entirely -- available from Ops 15, stationed at your station's Command Center rather than crewed onto a ship or sent on Away Missions. Assigning one there activates fleet-wide buffs across combat, economy, and ship-specific areas: examples include Phase Bank Upgrades (more Energy Weapon Damage), Alloy Metallurgies (cheaper Tritanium ship upgrades), and Unimatrix Training (more damage against Solo and Group Armadas). You can freely choose Kirk or Spock as a one-time free unlock through a mission; every other Fleet Commander comes as shards from daily gifts, events, or the Offers tab, and once you've built the Vindicator ship you additionally unlock daily shard claims for other Fleet Commanders. Improving one spends Skill Points, Commander XP, and Commander Credits on leveling, ranking up, and unlocking their skill tree.
Active Abilities are a newer, more powerful layer on top of ordinary Fleet Commander buffs -- a fleet-wide effect you trigger on demand rather than a passive stat bonus, gated by a cooldown so it's only live for a few minutes at a time. Crucially, it only works if that specific Fleet Commander is slotted in the First Seat -- the leftmost of your two Command Center slots -- so simply owning the right commander isn't enough if they're parked in the second slot. Gabriel Lorca was the Fleet Commander who introduced this mechanic: his Active Ability is a time-limited fleet-wide damage buff, unlocked at Combat Tree level 5, built around exactly the kind of short, high-stakes burst window the ability format is designed for.
Khan Noonien Singh is a Fleet Commander built primarily for PvP -- Arena, Station Defense, and general PvP combat all benefit from his buffs, and he adds a PvE angle too, boosting Apex Shred against non-Armada hostiles. He's built around three skill trees (Visionary, Extremist, Overseer) mixing critical damage, critical chance, Apex Barrier and Shred, and Isolytic Damage and Defense, with his own Active Ability, Wrath of Khan, delivering a large temporary critical damage spike. You can source him free through Khan Quests -- special, high-value Priority 1 Away Team Assignments unlocked in the Away Teams Research Tree -- or by defeating the Champion hostiles added with the Augment Faction expansion.
Benjamin Sisko is a hybrid Fleet Commander built around Outpost Combat and loot efficiency, with real value in general PVE and PvP on top -- sourceable from day one through the Outpost Refinery, so newer players don't have to chase a rare unlock to start using him. His kit spans three skill trees. Tactician (mostly always-on buffs) raises Isolytic Damage against non-Armada hostiles for Federation, Klingon and Romulan Grade 5+ ships, adds a huge Weapon Damage bonus against Outposts and their Retaliation hostiles, and includes his Active Ability: an on-demand fleet-wide Hull Health repair of up to 25% that doesn't apply to ships already in combat. Combatant leans PvP, raising Hull and Shield Health for the same three factions and adding large Apex Barrier bonuses against players under specific conditions (Morale, Hull Breach, Burning). Mechanic is the economy tree, boosting Cargo capacity, Mining Rate, positive reputation gain from Federation/Klingon/Romulan hostiles, Solo Outpost Credit and Component loot, and Outpost Fleet Size -- letting you field one extra ship in every Outpost engagement. Because so much of his kit is conditional on Federation, Klingon or Romulan Grade 5+ ships specifically, he's a stronger pick if that's already the core of your fleet than if you're flying mostly Independent or faction-specific hulls.