ICAO Annex 11
2.3 Divisions of the air traffic services

The air traffic services shall comprise three services identified as follows.

2.3.1 The air traffic control service, to accomplish objectives a), b) and c) of 2.2, this service being divided in three parts as follows:

a) Area control service: the provision of air traffic control service for controlled flights, except for those parts of such flights described in 2.3.1 b) and c), in order to accomplish objectives a) and c) of 2.2;
b) Approach control service: the provision of air traffic control service for those parts of controlled flights associated with arrival or departure, in order to accomplish objectives a) and c) of 2.2;
c) Aerodrome control service: the provision of air traffic control service for aerodrome traffic, except for those parts of flights described in 2.3.1 b), in order to accomplish objectives a), b) and c) of 2.2.

2.3.2 The flight information service, to accomplish objective d) of 2.2.

2.3.3 The alerting service, to accomplish objective e) of 2.2.

Air Traffic Services (ATS) is the overall framework. It comprises exactly three basic services:

• Air Traffic Control Service (ATC)
• Flight Information Service (FIS)
• Alerting Service (ALRS)

Important note: ATC is one of these three services. ATC is then internally divided into three parts: Area Control, Approach Control, and Aerodrome Control.

ATS = the complete system (ATC + FIS + ALRS)
ATC = one specific service within ATS

ATS = 3 basic services
---> ATC + FIS + Alerting

ATC = 3 subdivisions
---> Area + Approach + Aerodrome

Do not confuse the three Air Traffic Services with the three parts of ATC.