The PO# setting defines whether invoice PO# is visible and required.
If the Visible check box is selected, then your team can enter PO# while creating an invoice of the given type...
...as well as change PO# for existing invoices of this type.
If the Required check box is also selected, then your team must enter PO# while creating invoices of the given type.
If the Visible check box is not selected, then your team cannot enter invoice PO# because the PO# field is not shown while creating an invoice of the given type...
...and there is no way to change PO# for existing invoices of this type because the Change PO# option is not available.