Technical Notes 
In the flow logic of the screen, the required fields for E messages must be enabled for input by the FIELD statement in front of the module in question. After the user has corrected the entry, the module is processed again automatically from the beginning.
Messages within modules are generally triggered at the event PROCESS AFTER INPUT. Only I messages and S messages are useful at the event PROCESS BEFORE OUTPUT.
If several fields/modules are chained on the screen (CHAIN), the entire chain and therefore all modules are processed again.
If a field is contained in several chains and an error occurs, all chains which are in front of the module reporting the error are checked for this field and processed again, if found.