Official rule text
78.5 STEP 3—ROLL DICE: Each player rolls one die for each ship a Hits can only be assigned to units that are on the same planet as the units using the “Space Cannon” ability. they have in the active system; this is called a combat roll. If a unit’s combat roll produces a result that is equal to or greater RELATED TOPICS: Active System, Attacker, Defender, Destroyed, than that unit’s combat value, that result produces a hit.
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Source: Twilight Imperium: Fourth Edition - Living Rules Reference: Prophecy of Kings
Expansion: Prophecy of Kings
Version: 2.0
Reference: Rule 78.5