Headspace is the distance between the face of the bolt and the base of the cartridge case when it is fully seated in the chamber.
I’m not sure this is right. Or else I have understood it wrong over the years. I’m not a reloader.
That is correct. For rounds that headspace on the shoulder, the base of the cartridge may not be on the bolt face if the headspace is off. When fired, the case will stretch until the base of the cartridge hits the bolt face and the stretching could cause a case rupture.