Official rule text
78.5 STEP 3—ROLL DICE: Each player rolls one die for each ship 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 than that unit’s combat value, that result produces a hit. a If a unit’s combat value contains two or more burst icons, the player rolls one die for each burst icon instead. b If a player has ships that have different combat values in the active system, that player rolls these dice separately.
View official source
Source: Living Rules Reference and FAQ - Prophecy of Kings
Expansion: Prophecy of Kings
Version: 2.0
Reference: Rule 78.5