I would add that when you get near the check-in counters, there are roped off lines that you pass through before you enter the area where the counters are. There are uniformed KLM agents that will greet you as you enter the lines and let you know whether you are eligible to use the Sky Priority check-in. If not, there is another roped off line to enter the regular check-in area. You will be checked-in by KLM employees, even though your flight will be on Delta metal.