The forehead support for the original ResMed Ultra Mirage nasal mask is the "t-bar" without a forehead pad. This forehead support is the one that was designed for the first generation of Ultra Mirage nasal masks. It is compatible with some other ResMed masks (meaning that it will fit onto some other masks, like the Mirage Activa or the Ultra Mirage Full Face mask) but it is designed for the older style forehad pads and doesn't feature headgear release clips.
Forehead supports can be used with nasal masks to reduce pressure points on the face. One common pressure point is the bridge of the nose. If you feel discomfort on the bridge of your nose while wearing your Mirage Activa nasal mask, then increase the angle of the forehead support to move the mask away from the bridge of your nose.