For tasks that are not complete, the Stop field is the "complete through" date. For example, I create a task beginning on October 20, 10 days duration with a finish date of Oct 31. I set a status date of yesterday Oct 22 at 5:00 pm and use the Update Project command to update the task as of the status date. The Stop date = status date - the complete through date. The actuals (actual work and actual duration) are marked the appropriate % complete and % work complete through Oct 22.