Aerodrome Control Tower (TWR) shall provide:
a. Aerodrome control service;
b.Flight information service;and
c.Alerting service.

Aerodrome control may be divided into:
a. air control;and
b. ground (movement) control.

^^Air control shall be reponsible for the provision of service to aircraft flying in the vicinity of aerodrome and to aircraft taking off or landing. it shall have absolute authority over all movements on runways and their access points.

^^Ground (movement) control is established for the provision of services to aircraft,pedestrians and vehicles in the apron and manoeuvring area on than runway.

^^Taxi clearance
shall be issued to control the movement of aircraft taxiing on the manouevring area. Clear, concise and precise taxi instruction shall be given particularly with wide-bodied aircraft where the pilot's visibility form the flight decs is limited.

source by Air Traffic Control Training book