why are we using SET_OF_BOOKS_ID in po_distributions_all table

since org_id is using in po_distributions_all table still why are we using SET_OF_BOOKS_ID in po_distributions_all table

what is the value of org id

what is the value of org id in PO distribution table ? Isnt it always the OU in which PO is created SOB is the book of the org to which materials are charged