A purchase order, or PO, is a document a business issues to approve a purchase from a supplier. For a trade, it usually comes after your quote: the client accepts the price, raises a PO for that amount, and sends you its number. The PO is the client's internal permission to spend. Their accounts team matches your invoice to it before paying, so an invoice without the right PO number, or for more than the PO allows, is rejected or put on hold. Property managers, strata managers, schools, councils, builders and most larger businesses work this way.
A PO usually has a number, a description of the work, an amount, sometimes a site or job reference, and the client's terms printed on the back or attached by link. Those terms can include payment periods and conditions that differ from yours, so read them on the first PO from each new client.
In practice
An electrician quotes a property group $2,640 to replace a switchboard at one of its sites. The client raises PO 4471 for $2,640. On the day, the electrician finds two circuits that need new cable and does the extra work, $380, without asking. The invoice for $3,020 is rejected by accounts payable because it exceeds the PO, and it sits for six weeks while the site manager raises a second PO and the invoice is reissued in two parts.
The right order of events is: find the extra work, quote it, wait for the PO amendment or a new PO, then do it. It is slower on the day and much faster to get paid. The same applies to small jobs under a standing PO: if the PO is for "call-out and up to two hours", a third hour needs approval first.
Keep POs with the accepted quote they relate to, and copy the PO number, exactly as written, onto the invoice. The guide to commercial clients and purchase orders covers supplier set-up and payment terms.
In Quote Desk
There is no purchase order field on a quote. If the client gives you a PO or job reference before you quote, put it on the first line of the note so it travels with the quote, and copy it onto your invoice when the job is done. The quote builder page shows what a quote carries.