Hi THOR,
If you go to t/a SE80 and select Package UJ you'll see all BPC related programs, functions, tables, etc. Subclass UJK is for script processing; there you can see Class CL_UJK_RUN_LOGIC and Method EXECUTE_LGX that interprets script. That Method is pretty straightforward and you can find there what it calls when it finds RUN_PROGRAM in the script.
After a couple years of debugging scripts you'll know all those methods from the top of your head
.
Regards,
Gersh