I was always told as you were, seniority is for start time and vacations for PT. But at least at my hub (again, could vary by building/supplement language), HR has a preferred job list. You could put your name down once you passed your 30 days. Small sort, irreg train driver, hazmat responder and high value clerk were the ones available. When jobs opened up, they are supposed to go to those lists first. Doesn't always happen, but supposed to. Have had people placed there after filing grievances if needed.
If no one is on the list, supervisors have the right to staff the operation as necessary.