Official rule text
21.7 A player cannot spend commodities unless otherwise specified; a player can only trade them during a transaction. b When producing a fighter or infantry unit, a player can use a fighter or infantry token, as appropriate, from the
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Source: Twilight Imperium: Fourth Edition - Living Rules Reference: Prophecy of Kings
Expansion: Prophecy of Kings
Version: 2.0
Reference: Rule 21.7