Two types of graceful restarts are defined: planned and unplanned.
A planned restart would occur if the software module for OSPFv3 was upgraded, or if the router operator decided to restart the OSPFv3 control function for some reason. The router has advance warning, and is able to inform its neighbors in advance that OSPFv3 is restarting.
An unplanned restart would occur if there was some kind of system failure that caused a remote reboot or a crash of OSPFv3, or a failover occurs. As OSPFv3 restarts, it informs its neighbors that it is in the midst of an unplanned restart.
You can decide to configure a router to enter graceful restart for only planned restarts, for only unplanned restarts, or for both. Also, you can separately decide to configure a router to be a helper for only planned, only unplanned, or for both kinds of restarts.