A purchase order represents items that you purchase from a vendor. It may include materials which are reimbursable, labor items, equipment items, expense items, or even discount items.
Expenses (also called job expenses) are a special type of purchase order with some added functionality. They can be added from the job's completion page, or they can be added from the Create New Purchase Order page. Expenses added from the job completion page, along with purchase orders created within the job's visit(s)contribute to job costing within SingleOps. Purchase orders created and associated to the job do not affect the job costing inside SingleOps - although they sync to QuickBooks in exactly the same manner.
Bills are the QuickBooks' equivalent to SingleOps' purchase orders, and thus to SingleOps' expenses. You can, however, configure purchase orders and expenses in SingleOps to not sync to QuickBooks if you so desire.