ATA 21 in one line
Pneumatic air is conditioned by packs, distributed and recirculated through the cabin, adjusted for temperature demand, while a separate pressure-control loop regulates outflow.
Use this as a refresher
Place the idea inside the full system chain: source/input → control → physical response → sensing/indication → consumer/result. If the quick note seems to conflict with the aircraft in front of you, stop and confirm configuration, effectivity and current approved data.
Maintenance boundary: this is a study reminder, not a task instruction.