Where the system is only lined with a single skin of plasterboard, the flat strap will be fixed direct to the studs and must be the FS90W. 

For systems which are double boarded, flatstrap can be installed in between the board layers (positioned to pick up the outlayer horizontal joint) and this can be the FS50Rx strap. However if positioned against the studs it must still be the FS90W strap even if double boarded. This should still be positioned to pick up the outerlayer horizontal joint.