It is an area of the Lufthansa lounge that has a small sign "reserved for Senator/First Class customers"- otherwise business and Senator are the same lounge. It is quite new (not more than 18 months) and follows the newish Lufthansa lounge designs. I think the hours it is open only coincide with Lufthansa departures so if you have a long layover it may not be open- in which case there is a Singapore Airlines lounge immediately next door or alternatively take the elevator one level higher above and there is a Plaza Premium lounge which you can pay entry into.