Rails- :dependent => :destroy VS :dependent => :delete_all

In rails guides it's described like this:

Objects will be in addition destroyed if they’re associated with :dependent => :destroy, and deleted if they’re associated with :dependent => :delete_all

But what's the difference between being destroyed and being deleted?

The difference is with callback.

The :delete_all, made directly in your application the delete by SQL :

DELETE * FROM users where compagny_id = XXXX

With the :destroy, there are an instantiation of all of you children. 

So, if you can't destroy it or if each has their own :dependent, its can be called.