T-OPER-PGM-OP-MODE is known as the PROGRAM OPERATING MODE flag. It tells your program whether tablesONLINE/CICS was operating in B (Browse) or E (Edit) mode when the exit was invoked.
This is commonly used in an exit program called with an exit taken before a table is opened; it is used for security or integrity checking. If the flag contains E (Edit), then the table will be opened for write access. If the flag contains B (Browse), then the table will be opened in read-only mode.