Following Arden's instructions, I suppose that BAdI just sends out message requests to outbound correspodence queue to be processed by the program RCRM_LOY_OUTBND_PROCESS_QUEUE.
The BAdI uses your personalized mail form as mail body and CRM_LOY_MSH_PPF_FORM adobe form as attachment (from what I understand from the code, haven't tested it out myself yet)
So in case you would like to make some modifications in the adobe form, you can take CRM_LOY_MSH_PPF_FORM as reference ( copy it) and make your own form.
The BAdI has got another method for smartform processing in case you want to send a smart form as attachment rather than an adobe form. (But the method has no sample code, it's empty)
Hope this brings some clarity 