Official rule text
89.1 STEP 1—ACTIVATION: The active player must activate a system that does not contain one of their command tokens. a To activate a system, the active player places a command token from their tactic pool in that system. That system is the active system. b Other players’ command tokens do not prevent a player from activating a system.
View official source
Source: Living Rules Reference and FAQ - Prophecy of Kings
Expansion: Prophecy of Kings
Version: 2.0
Reference: Rule 89.1