Canon EOS-1D X Mark II DSLR Camera (Body Only) - Manual (Page 128)

3 Customizing AF Functions
128
If [ EOS iTR AF (Face priority) ] or [ EOS iTR AF ] is set, the camera may
take longer to focus than when [ Disable ] is set.
Even if you set [ EOS iTR AF (Face priority) ] or [ EOS iTR AF ], the
desired result may not be obtained depending on the shooting conditions
and subject.
Under light so low that the EOS-dedicated, external Speedlite emits the
AF-assist beam automatically, AF points are selected automatically
based only on AF information.
Face detection may not work if the face is small or under low-light
conditions.