If not specified, the po_line_number field is left blank when creating po_lines via POST /orders/<poId>/lines.
The po_line_number should be the <po_number>-<sequential #>. So if the po_number of the associated purchase_order is: AB12345 and this is the 3rd po_line to be added to it, the po_line_number would be AB12345-003
- po_line_number takes for form: `<po_number>-<sequential number between 1 and 999>`
- po_line_numbers are always automatically assigned. A user can't specify a po_line_number.
- if po_lines are created and later removed, gaps may appear in the sequence. This is acceptable. New po_line_numbers will continue to be assigned from the next number in the sequence (as opposed to trying to fill in the gaps)