Energy is your health and your ammo. The ring around your ship shows what is left.
Greens power you up and add +10 bounty. A kill adds +50. Everyone starts at 500; when you go down you drop back to 500 and lose your upgrades.
Whoever shoots you gets your bounty as points. Rich pilots attract hunters, and the leader wears a crown.
Arcade: against the clock, with respawn and combos. Last Man Standing: limited lives, last pilot flying wins. Change enemies, time, lives and more under Game options (custom rules do not count for records).
Team CTF: two teams of six start in their own base and fight over the flag in the middle. Carry it into the enemy base to score (first to the capture limit, or the lead when time runs out; a tie goes to overtime). Drop it when you are shot down, or throw it (Throw button, Q on a keyboard): it flies like a bomb, bounces off walls, and counts as a capture if it lands in the enemy base. Bases are safe zones, so the fight happens around them.
In the other modes the green circle in the middle is a safe zone: no shooting there.