Official rule text
78.6 STEP 4—ASSIGN HITS: Each player in the combat must the capacity of that player’s ships in that system, that player choose and destroy one of their own ships in the active system must choose and remove any excess units. for each hit their opponent produced. RELATED TOPICS: Capacity, Fleet Pool, Opponent, Sustain
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Source: Twilight Imperium: Fourth Edition - Living Rules Reference: Prophecy of Kings
Expansion: Prophecy of Kings
Version: 2.0
Reference: Rule 78.6