EXPOSURE and EXPOSURn
In principle, EXPOSURn is an ACIS header keyword whose value for CCD_ID = n (where n = 0, 1, 2, ..., 9) includes not only the dead-time correction used to compute the value of LIVETIME, but also all other adjustments to the effective observing time. In practice, the values of EXPOSURn and LIVTIMEn are identical. The value of the keyword EXPOSURE is the same as the value of the EXPOSURn of the CCD at the aimpoint of the telescope.
If an observation uses interleaved mode then there are separate values of EXPOSURn and EXPOSURE for the short and long frames.
The observation times entry and the chandra times help file explain how the various exposure keywords are related.