The document defines several key airport functions and services: 1) Check-in counters are where passengers check in, drop off baggage, and receive boarding passes. 2) Immigration controls people entering a country while customs controls goods people bring with them. 3) An apron or ramp is an area where aircraft can load/unload passengers and cargo, refuel, park, or get maintenance. 4) A bay is an area where aircraft park, load, unload, refuel, or board.