First of all, purchase orders are separate from your customers, and they have no impact on your financial position or performance. They are simply a professional way to order materials from suppliers. They do make it easier to produce a enter a purchase invoice when you are billed by your supplier. The purchase invoice, in turn, is meant to be used when you buy on credit and pay later. It will produce an account payable. If you pay immediately upon delivery or presentation of your supplier’s invoice, you should just Spend Money from a bank account.
Disbursements are meant to be used when you make expenditures on behalf of your customer that you expect to be reimbursed. Examples are travel, supplies or materials directly applicable to a job, etc. Think of them as billable expenses. (Some of us, in fact, have advocated for changing the tab’s name to Billable Expenses. @lubos is still working on this capability, so we’ll see where he goes with it.) The advantage of Disbursements is that things that go in there are not meant to be recorded as expenses of your business, like rent, insurance, office supplies, and so forth. They are things that the client could have paid for directly, but it was more convenient for you to do it for them.
For more general things, like vehicle fuel, materials that might or might not all end up in a client’s project (like consumable supplies, such as reels of cable, boxes of fasteners, reams of paper, rolls of tape, etc., just Spend Money from a bank or cash account and allocate the cost to the appropriate expense account. If you want, you can enable Tracking Codes and keep track of individual projects that way.