Events can be locked by all people who has permissions to lock events in worktime roles.
Work time roles can be edited and for example supervisors can be given permissions to lock subordinates events. With Can lock event dates permission in High-level permissions, gives permission also to lock events at the same time they are approved.
Unlocking event locks is in High-level permissions (usually only main users). Permission gives possibility to unlock and edit already approved and locked events. Usually employees are not given permissions to unlock locked events.
If only supervisors are wanted to give permission to unlock events, it is recommendable to create supervisors own worktime role where supervisors have same permissions than in the orginal worktime role and additionally permission to unlock locked dates.