AHU is the abbreviation of air handling unit and FCU is the abbreviation of the fan coil unit. AHU is used for small, medium, and very large capacity applications, however, FCUs are used for small capacity applications. AHUs are used for central air conditioning, however, FCUs are used normally for local applications.  AHUs can take fresh air and condition it, however, FCUs are normally not taking outside air and only used for recirculation & conditioning purposes.