How does the LedgerTrans table get populated?

I'm reading that I should work with the classes LedgerVoucher, LedgerVoucherObject and LedgerVoucherTransObject but I need more detail...how does it get populated? For instance, if I created a new field in the LedgerTrans table to store the journal number, how do I post from an inventory journal or from a GL journal?

This is what I've done: I declared my field and create the parmJournalID() methods. I place my breakpoint in LedgerVoucher.addTrans() and the LedgerVoucherObject .addTrans() but the journalID seems to have already been passed when the parmJournalID() method is called. Please help!