The T-TBLX-EXIT-BA-IND parameter denotes whether the exit is being called Before or After (values B or A respectively) the operation described above (that indicated by the I/O indicator) is performed. This value is specified when you are defining a View in tablesONLINE/CICS.
Exit programs should treat these three indicators (STIF, I/O, Before/After) as strictly read-only data. They can be accessed either individually or as a three character string.