Stage 1
These upgrades are the very basic setups to gain initial power, mostly through oem software recallibration.
Stage 2
Intake and exhaust upgrades make sure the engine will breath easier and allow the tuner to add more timing and fuel to achieve higher performance levels.
Stage 3 and above
These will take your car to a next level, ranging from basic cooling and turbo upgrades to full engine builds and custom software callibrations or standalone engine management.