When does fallback setting take effect?

[Hongming Xiao]

All,

The grub manual says:

fallback

If this variable is set, it identifies a menu entry that should be
selected if the default menu entry fails to boot. Entries are
identified in the same way as for ‘default’ (see default).

My question is: how does grub know whether the system "fails to boot"?
Let's say in grub.conf default is set to 0 and fallback is set to 1,
will system boot into 1 in following scenarioes? If yes, from where
grub learn what happened?

a) kernel 0 crashed during startup
b) kernel 0 failed to start some service during startup?
c) kernel 0 crashed at any time
d) cycling power while kernel 0 startup?
....

Thanks,
Huang Zhong