Assuming you are talking about using a FAST lane when empty, my experience has been that it is a grey area. For going into Canada I was given a card that only had my carriers I.D. on it. I was told this was for using the FAST lane coming in empty and with dunnage. Some officers were fine with it, others said not to use the FAST lane. I decided to not use the FAST lane empty going either way to avoid any confusion.