Fleet Assignment

Fleet Assignment is the process of assigning a specific member of a House to command a fleet in combat.

Codex
"When a house mounts an attack or defense, a fleet commander (and, optionally, supporting staff) must be chosen from the house members assigned to Military. A fleet operation can take several weeks or months, given the time taken to travel between systems, so a commander sent to lead a fight on one side of a faction will not be available to be assigned elsewhere (or defend at home) for a turn."