Canon EOS 70D Instruction Manual page 90

f: Selecting the AF OperationN
AI Focus AF for Switching the AF Operation Automatically
AI Focus AF switches the AF operation from One-Shot AF to AI
Servo AF automatically if a still subject starts moving.
After the subject is focused in One-Shot AF, if the subject starts
moving, the camera will detect the movement, change the AF
operation automatically to AI Servo AF, and keep tracking the
moving subject.
AF-Assist Beam with the Built-in Flash
Under low-light conditions, when you press the shutter button halfway,
the built-in flash may fire a brief burst of flashes. It illuminates the
subject to enable easier autofocusing.
90