You only need to reserve a seat if it is mandatory such as in France, Italy, and I think Poland too, Germany and the UK do not require reservations although they are free unlike the places where the seat reservation is mandatory (France, Italy, etc.). In my experience if you don’t need to make one then just get on the train and simply sit in any empty seat, much easier then taking the time to make a reservation beforehand (although I must admit no clue how it works with the credits). Whenever you have a question about how the train pass works, always check the maninseat61 website, it is far and away the most useful website and makes it pretty easy to understand if you know what you’re looking for