This ACIS header keyword is the "dead time" correction factor. It represents the fraction of the total time per frame that contains useful X-ray data:
The value is between 0 and 1, and depends on the operating mode used for the observation. The value for the standard, full-frame, mode is 0.987337. If an observation uses interleaved mode, then there are separate values of DTCOR for the short and long frames.