Jobs and Accounts can be selected on Purchase Order details, and will be posted to job details when the GL account used is either an expense or cost of goods account. PO Details with inventory accounts will be ignored with respect to jobs.
One way to select a job and account for a purchase order is to open the Job tab and do so here. After the selection, each detail line of the PO will be populated with the selected job and account.
You can also select a job and account for each detail line of the purchase order without visiting the Job tab, which is typical when many jobs will be referenced on this purchase order. If there is a job/account set for a non expense detail line, then it will be ignored for the information posted to Job Costing.