Official rule text
23.4 UNITS: Units are limited to those included in the game, except game effect that allows them to replenish commodities. for fighters and ground forces. c If a game effect instructs a player to convert a number of a When a player would place a unit, if there are none of that their own commodities to trade goods, those trade goods type left in their reinforcements, that player can remove are not treated as being gained for the purpose of triggering a unit from any system that does not contain one of their other abilities. command tokens and place that unit in their reinforcements.
View official source
Source: Twilight Imperium: Fourth Edition - Living Rules Reference: Prophecy of Kings
Expansion: Prophecy of Kings
Version: 2.0
Reference: Rule 23.4