What is Dept. EP, Dept. B and Suite in the following addresses?

Often, when you respond to a particular department, you are really telling them where you saw their particular advertisement. That is, one magazine might list Dept A, and another magazine uses Dept B. They all go to the same place, but they find out which magazine (or whatever) is bringing in the most responses.

By the way, if you are answering an advertisement like that, it is quite possible they are not a conventional publisher. That is, instead of paying you for what you write they expect you to pay for publishing it.

Suite could be a room, or an address with a number of rooms. It's usually used when a number of businesses occupy the same street address.