All the transactions that charge expenses to a job are entered in modules other than the Job Costing System itself. The month end posting of these transactions (vouchers, debit memos, inventory adjustments, payroll distribution) are all done with their respective closing. If you were to close a period without running Post Jobs / Work in Process, (accessed from the Select, Job Costing menu), you would be expensing each job perpetually and not building an asset for future billing. So run this process before you run the trial close process on the Closing menu.
Post Jobs / Work in Process accumulates all the expenses that are posted to a job and moves the accumulated expense to the work in process account that is specified for each job. This creates an asset which is billed out to a customer at a later date. When a job is closed this process moves the accumulated expenses of previous periods out of the work in process account back to the expense accounts. The customer can then be billed.