I have a question about the connection between PO and BILL.

1.
Usually we make PO and when we receive the items, we make BILL. Once a BILL is created for every item on the PO, it deleted from the OPEN PO report.

Recently, when reviewing OPEN PO REPORT, There are some POs were in OPEN PO, even though all of them were billed.

Please advise to delete these PO in open PO.

2.
And when we create a BILL, a V box appears in BILLABLE for some BILLs, but not for some BILLs.

What is the difference?