The constraints on sequences of shifts are used to control the different types of shifts that employees would work consecutively. For example, if employees share the night shifts and all have to work them, you may need to control the maximum number of consecutive night shifts.
Sequence constraints can also apply to mandatory sequences. For example, it is mandatory to have 2 days off after 3 night shifts. You may also have forbidden sequences where employees can’t work a day shift after an evening shift.
In short, sequences control the different combinations of shift types a planner can use to build a schedule. These controls are usually in place for both fairness and fatigue. When an employee works on all shifts, any schedule they get will be difficult and full of mistakes from the employee’s perspective.
Planners need to protect themselves by stating clearly what sequences are controlled, for fairness and/or fatigue.