Departments are like Classes...except when they're not?

In the documentation and in the NetSuite GUI, Departments, Classes & Locations are all made to seem the same.  I can't speak for Locations as I haven't used those extensively, but it seems to mean that Departments & Classes don't behave in quite the same way.  Can others confirm the differences I note below?

First, Classes. We've been using these for a while, and there are certain behaviors which I find very helpful.  For one, if you apply a Class to an Item, that Class is automatically (and "retroactively") applied to all Transactions which contain that Item. In fact, when it comes to search results, its applied at the line Item level. A user without access to a certain Class can't load any Transactions where that an Item of that Class has been sold.  If he does a search for sales of a different Item which he does have access to, he'll be able