Official rule text
75.3 A player can spend a trade good as if it were one resource. the “Space Cannon” ability of their PDS units that are in RELATED TOPICS: Exhausted, Planets, Producing Units, Trade systems that are adjacent to the active system. The hits are
View official source
Source: Twilight Imperium: Fourth Edition - Living Rules Reference: Prophecy of Kings
Expansion: Prophecy of Kings
Version: 2.0
Reference: Rule 75.3