The DIF_GROUP flag, if specified with the first DI_RADIOBUTTON control, and groups sequentially declared radio buttons:

// color selection dialog in FAR Manager:
...
// first group of radiobuttons
DI_RADIOBUTTON,6,3,0,0,0,0,F_LIGHTGRAY|B_BLACK|DIF_GROUP|DIF_SETCOLOR|DIF_MOVESELECT,0,"",
DI_RADIOBUTTON,6,4,0,0,0,0,F_BLACK|B_RED|DIF_SETCOLOR|DIF_MOVESELECT,0,"",
...
// second group of radiobuttons
DI_RADIOBUTTON,21,3,0,0,0,0,F_LIGHTGRAY|B_BLACK|DIF_GROUP|DIF_SETCOLOR|DIF_MOVESELECT,0,"",
DI_RADIOBUTTON,21,4,0,0,0,0,F_BLACK|B_RED|DIF_SETCOLOR|DIF_MOVESELECT,0,"",
...