I only know them in the context of English exams where "seen" means you've seen and studied the text before e.g. A book or poem and you're asked a question on it that you haven't seen before. "Unseen" are questions about something you've not seen/studied before so a completely new poem for example. Hope that helps!
On our access course we had a 'seen' exam. It basically meant we could see the questions before and revise the answer. We'd just have to remember the answers for when we sat the exam. Unseen is basically random questions from your subject that you won't know of beforehand.