It's the foreign key that links to FINCTRL.FINCNTRLID. The FINCNTRL table holds the data for the Cost Management application.
Basically:. Cost Management holds the budget day for a project. Each entry in the Cost Management application. Can have one or more work orders referencing it.
To set this up, you need to add two non-persistent fields to the work order screen, which when filled in by the user, result in the FINCNTRLID field being set. There's an article on the IBM site on what to do.