In some unionized environments, employee seniority is used as a sorting list for offering the best schedule or the any shift that has additional pay. Seniority can be calculated in many different ways. The usual way is to use the employee hire date. In some European countries, seniority is determined based on your age and not on the day when you were hired.
Any business that sorts on seniority is using the criteria as an unbiased accepted form of tie breaking and distribution of employee satisfaction. For example, manufacturing industries with day, evening and night shifts will offer the shifts based on seniority. The most senior employee will pick an available shift or express a top preference that will be followed by the planner. All employees may want the day shift, but only the most seniors will actually get the day shifts.
Seniority is used in many areas, such as shifts, activities to do (some tasks are harder than others and the most senior employees avoid them), overtime offers, weekend premium shifts, etc.
Reverse seniority is also used to force employees to come in to work. For example, if all employees refuse the overtime but employees are still missing, then employees are forced back to work using a reverse seniority order (the most junior employee being the first called back to work).