LIVTIMEn and LIVETIME
LIVTIMEn is an ACIS header keyword whose value for CCD_ID = n (where n = 0, 1, 2, ..., 9) is the total amount of time the CCD was actively observing a source (i.e. the "live time"). This time excludes the "dead time" associated with the time it takes to transfer charge from the image region to the frame store region (0.04104 s per frame) and the time spent performing preflushes (if they are necessary):
Since the values of ONTIME can differ from CCD to CCD, there is one LIVTIMEn keyword for each CCD used for an observation. The value of the keyword LIVETIME is the same as the value of the LIVTIMEn of the CCD at the aimpoint of the telescope. If an observation uses interleaved mode then there are separate values of LIVTIMEn and LIVETIME for the short and long frames.
The observation times entry and the chandra times help file explain how the various exposure keywords are related.