A purchase order is the moment a cost becomes real. The cabinets are ordered at $10,500, the supplier confirms, and the job now owes $10,500 no matter when the invoice shows up. Larger contractors issue formal POs with numbers the supplier prints on the invoice; small contractors "place the order" by phone or email, which is the same commitment without the paper. Either way, the job's forecast should move the day the order is placed, not the day the bill is paid.
A worked number
Week two: cabinets ordered, $10,500. Week nine: cabinets delivered and invoiced. A job costed from invoices shows $10,500 less cost for seven weeks; a job costed from commitments shows the right number the whole time. Across a $48,000 budget with three such orders, the invoice-only view can be $20,000 optimistic in the middle of the job.
The mistake it hides
Paying an invoice that doesn't match the order. A PO with a price on it is the reference the invoice is checked against; without one, a supplier's price change or a quantity error goes through unnoticed, and the job pays for it.
In Obra
Obra doesn't keep purchase orders as records. When you place an order, put its amount into the project's forecast review as remaining to spend, so the expected final cost counts it from that day; when the invoice is confirmed, the cost moves from remaining to recorded and any difference from the order shows up in the forecast. The committed costs guide covers the forecast this feeds.