Look at statements GET and CALL BADI and transaction SE18.
Reference: Look in OSS notes for Enhancement Framework : Enhancement Technologies , Business Add-Ins (BAdIs), Definition of BAdIs. (or BC425 training course)
Hint: For oldest SAP version also look at CALL CUSTOMER-FUNCTION or CUSTOMER SCREEN and SMOD transaction. For intermediate version SE18 but in caller use some call method cl_exithandler=>get_instance changing <instance> and then some <instance>->badimethod
Regards,
Raymond
