System-level exits are used to provide an opportunity for special user validation and to set system defaults.
For the system sign-on exits, SIB and SIA, tablesONLINE/CICS system initialization has not been completed. Message stacks, for example, are not yet established, so standard tablesONLINE/CICS message processing is unavailable. In the case of SIB, the exit, which is specified in TBOLCNST, controls the access to tablesONLINE/CICS. The SIA exit occurs after TBOLACT application control processing is completed; this exit can be used to perform tracking of user-specific usage of tablesONLINE/CICS. Setting the BYPASS-ACTION-IND to Y or E prevents a user from accessing tablesONLINE/CICS.
For the System sign off exits, SOB and SOA, tablesONLINE/CICS system initialization has been completed. The SOB is the logical counterpart to the SIA exit. If a timer was activated at the SIA exit, here would be the place to inspect the timer and record if so desired. Setting the BYPASS-ACTION-IND at this time has no effect.