Using ServiceTrade and QuickBooks together, you can give your operations team (schedulers, technicians, etc) real-time visibility into job costs, AND make sure that the accounting controls that your business requires are being followed when your have to purchase materials to perform a job.
Normally, ServiceTrade will create customer and job records in QuickBooks at the end of a job's lifecycle, when you send an invoice from ServiceTrade to QuickBooks. However, if you need to use purchase orders to authorize parts purchases, you will need the job to be in QuickBooks earlier, before it's invoiced -- probably even before your technician arrives onsite! -- so you can associate purchase orders with that job.
Please note that you'll need to use the "Customer : Job" or "Customer: Location : Job" mapping for QuickBooks. See the "Customer mapping" section of this article for more information about how to set up your customer mappings. You'll also need to be using a version of QuickBooks Desktop that supports inventory management (this is required for you to make use of purchase orders). QuickBooks Online does not officially support job costing.
To send a job from QuickBooks to ServiceTrade, click the "Send" button at the top of the job page, then choose the desired QuickBooks Desktop connection from the list that appears. Just like with invoices, the job will appear in QuickBooks once your QuickBooks Web Connector has synced all new changes from ServiceTrade to QuickBooks.
Once the job is created in QuickBooks, you can can create purchase orders against it. When the job is eventually invoiced in ServiceTrade and the invoice is sent to QuickBooks, that invoice will be associated with the same job, so you'll be able to create gross margin and profitability reports that make use of the job-level cost and revenue information that you've tracked.
For more information about job costing in QuickBooks, this article on Intuit's Accounting blog has a great introduction.