Rf-t refers to the flexible wall types “GKB” and “GKF”. What do these abbreviations mean?

These two abbreviations describe the type of plasterboard that can be applied as siding materials on the metal studs of a light wall. "GKB" indicates standard plasterboards (type A according to EN 520); "GKF" boards offer superior fire resistance for the same thickness (F type according to EN 520).