Canon EOS 70D Instruction Manual page 107

19-point Automatic Selection AF
All the AF points are used to focus. This mode is set automatically in
Basic Zone modes.
When AI Servo AF mode is set with 19-point automatic selection AF or
Zone AF, the active AF point <S> will keep switching to track the
subject. However, under certain shooting conditions (such as when the
subject is small), it may not be able to track the subject. Also, in low
temperatures, the tracking response is slower.
If the camera cannot focus with the EOS-dedicated external Speedlite's
AF-assist beam, set the AF area selection mode to Single-point AF
(manual selection) and select the center AF point to autofocus.
When the AF point(s) light up, part or all of the viewfinder may light up in
red. This is a characteristic of AF point display (using liquid crystal).
If you set [8C.Fn II-9 : Orientation linked AF point] to [1: Select
separate AF points], you can set the AF area selection mode and
manually-selected AF point (or zone) separately for vertical and horizontal
shooting (p.373).
With One-Shot AF, pressing the shutter button
halfway will display the AF point(s) <S> that
achieved focus. If multiple AF points are displayed,
it means they all have achieved focus. This mode
tends to focus the nearest subject.
With AI Servo AF, the manually-selected (p.105) AF
point <S> is used first to achieve focus. The AF
point(s) achieving focus is displayed as <S>.
AF Area Selection Modes
107