Official rule text
89.4 STEP 4—INVASION: The active player may use their A system tile represents an area of the galaxy. Players place system “Bombardment” abilities, commit units to land on planets, and tiles during setup to create the game board. resolve ground combat against other players’ units.
View official source
Source: Twilight Imperium: Fourth Edition - Living Rules Reference: Prophecy of Kings
Expansion: Prophecy of Kings
Version: 2.0
Reference: Rule 89.4