When T-TBLX-EXIT-BA-IND (the Before/After INDicator in the EXITPARM copybook structure) is B, the exit program is being called before some specified action is executed, and on normal return tablesONLINE/CICS will take that action.
In most applications, these Before exit programs (TOB, TCB, TSB, IIB, IUB, INB, IMB, IDB, FIB, and FOB), validate data to help ensure that all will go well when tablesONLINE/CICS performs the action.