I would like to know if the Target usage parameter change something for h264 QSV encoder or if the parameters is only available for h264 SW encoder? For SW I see CPU difference but for QSV it is not easy to see the effects.
The answer is very dependent on the hardware (and implementation of hardware library). The number of 'unique' Target Usage settings can vary for each platform and video standard.
You can assume that Target Usage 1 is equal or higher quality than higher values, and that Target Usage 7 is equal of faster than lower values.