Official rule text
1.12 Some examples of an ability’s cost include spending resources, abilities provide players with perpetual effects. spending trade goods, spending command tokens, exhausting cards, purging cards, and activating specific systems.
View official source
Source: Twilight Imperium: Fourth Edition - Living Rules Reference: Prophecy of Kings
Expansion: Prophecy of Kings
Version: 2.0
Reference: Rule 1.12