Sure!
In the current version of SFML, they use the event WM_MOUSEMOVE which basicly returns x and y coordinates after the mouse have been moved and translated by the OS.
I am using the event WM_INPUT which returns raw data directly from the HID bypassing the OS changes. The HID can be either a mouse or a keyboard. I think there are other types of raw input devices, but I don't know any specific..
What the setRelativeMouseMode() does is that it's (en/dis)abling raw input mode. It has to be toggled on, else WM_INPUT events wont be fired at all. The raw data for the mouse contains the delta of the previous X coordinate of the mouse and the current X coordinate of the mouse. Same for Y. It also contains some information for the mouse button states, but I still doesn't know too much about it.
I hope that makes sense, because that is the best way I can explain it.