-local vs. -hook?

[Karl Berry]

Reading the Extending node in the automake 1.10.1 manual:

   However, various useful targets have a `-local' version 
   ...
   Some rule also have a way to run another rule, called a "hook"

What I am missing is an explanation of the difference (from the user's
perspective.  That is, when to use install-exec-local and when
install-exec-hook?  If it doesn't matter, then I think it would be worth
saying so explicitly.

Also:
(a) "Some rule" is not grammatical here; it should be "Some rules".
(b) Shouldn't it be "Some targets also have a way to run another target"?

Thanks,
Karl