status of a purchase order

hi

i want to know status of a purchase order (i.e. a PO is pendding , closed, invoice pending , complete like this).

there is report (t.code ME2N) which gives details of PO's

by giving INPUT to 4th SELECT-OPTIONS (tittled as SELECTION PARAMETERS), here in this many options are available .

i searched for these options , these get stored in table T160S ,But i am not able to under how corresponding to these values program is picking all related data from other tables like EKKO,EKPO.

how it is relating these tables.

plz help.