What is the difference between "unreasonable self-defense" and "true self-defence"?

The distinction between the two doctrines is pointed out in People v. Barton (1995) 12 Cal.4th 186, 199-200: "The sole difference between true self-defense and 'unreasonable self-defense' is that the former applies only when the defendant acts in response to circumstances that cause the defendant to fear, and would lead a reasonable person to fear, the imminent infliction of death or great bodily injury [citations]; unreasonable self-defense, on the other hand, does not require the defendant's fear to be reasonable. ([] Christian S., supra, 7 Cal.4th at p. 773.)"