Thank you for answering. Yes, I have read the documentation, and @optOut is the only available option in the Delivery and tracking (DeliverLogStats) table. When setting up the cube report, one can choose "distinct count", but this gives a very wrong number.
An example: I have an e-mail where 401 recipients clicked the opt-out link at least once (found by using a query on the tracking log table). The cube report shows sum @optOut = 489 which is the total number of clicks. But distinct count @optOut shows 14... Which is a number that makes no sense to me. I would have excpected the distinct count to be 401, but clearly I am missing something...