What is the difference between down-mouse-3 and mouse-3

["Ye Wenbin" <wenbinye@gmail.com>]

I wrote try to figure out how to set up mouse menu, after spending a lot of
time to to figure out why no action preform when click submenu, I found  
that
I should bind down-mouse-3 event rather than mouse-3 event. I can't find
where is the difference mentioned in elisp manual. Is there any documents
for the reason that submenu does not work for mouse-3 binding?

Here is my test code:
(define-derived-mode foo-mode text-mode "Foo"
   "Test major mode"
   )
(easy-menu-define foo-menu foo-mode-map "My own menu"
   '("Foo"
     ["Next Line" next-line t]
     ("Sub Menu"
      ["Previous Line" previous-line t])))

(define-key foo-mode-map [mouse-3] 'mouse-popup-menubar-stuff)
;; (define-key foo-mode-map [down-mouse-3] 'mouse-popup-menubar-stuff)

-- 
Best Regards,
Ye Wenbin