The TIME-TAG mode records the arrival time of each photon on the COS and STIS MAMA detectors. The TIME-TAG mode is the default observation mode for COS, but for STIS this mode needs to be actively selected. Opting for TIME-TAG observations with the STIS/MAMA detector has several advantages and a few limitations. These are fully described in the STAN of June 2014.
Note that when opting for the TIME-TAG mode, it is important to check that the observations do not violate any TIME-TAG specific count rates or data volume constraints. The ETC does not issue TIME-TAG specific bright-limit violations warnings. BUFFER TIMES need to be calculated with care as well with this mode. Details of the MAMA TIME-TAG mode of operations and guidelines to compute the BUFFER TIME are provided in Section 11.1.3 of the STIS Instrument Handbook.