Hi George,
the BAdI is easy to use. You can look into the interfaces with TA SE18 and will always get an import parameter which is called IM_DATA and a changing parameter where you can set the change. For example:
You want to change the assignment number (ZUONR):
Insert the following coding into the method ACCIT_ZUONR_EXIT to set the security ID into the assignment field:
CH_ACCIT_ZUONR_D = im_data-security_id.
CH_ACCIT_ZUONR_C = im_data-security_id.
The parameter CH_ACCIT_ZUONR_D is the assignment no. for the debit line and the parameter CH_ACCIT_ZUONR_C is the assignment no. for the credit line.
But be careful: The coding will be used in each posting indepentent from product type, company code etc.
Cheers
Robert