Perhaps one of the most important functions of the commanders is allowing the purchase and use of commander cards. After players have purchased and placed their commanders or space station, players may purchase a maximum of four commander cards at a cost of one energy per card. Players may only purchase commander cards if they have the corresponding commander in play. The text of the commander card will illustrate when the card may, or must, be played. Many of the Commander cards have energy costs that must be paid if the card is to be put into play.