PO history status texts

Hi All,

For subsequent debit and credit, the PO history is shown with indicator NB-L. Also, in some cases I can PO history with indicator SD-L (Subs. deb. log. IV). How is this status getting updated in PO history. Which transaction will update this, and could you please explain the process to be followed to get this status in PO.

Thanks in Advance.

Alex