Canon EOS 5D Mark III Instruction Manual (en)

 Setting the Time Code
If the frame rate setting is 6(29.97fps) or 8(59.94fps), the time code’s 
frame count causes a discrepancy between the actual time and time 
code. This discrepancy can be corrected automatically. This correction 
function is called drop frame.
: The discrepancy is corrected automatically by skipping 
time code numbers (DF: Drop frame).
: The discrepancy is not corrected (NDF: Non-drop frame).
Drop Frame
When the frame rate is set to 4 (23.976fps), 5 (25.00fps), or 7 
(50.00fps), the drop frame function does not take effect. (If 4 is set or [53: 
Video system
] is set to [PAL], [Drop frame] option will not be displayed.)