CPO usually means a Conditional Purchase Order where the buyer makes an offer conditional on the seller's acceptance of the buyer's terms and conditions. This sort of document is quite common in all sorts of commercial fields and is similar to a Contract Purchase Order.
Never be embarrassed to request the other party to specify exactly what they mean when they give you abbreviated terms.
Better safe than sorry because the other party might come up with an exotic meaning not in your favour after you pay money.
May the Gods of Trade ... Shangdi, un dieu, dio, Gott, Dios, Deus, Allah, alaha ... smile on us