References is used in sketcher mode only.It lets you use existing geometry in a better way.By adding references , sometimes you can resolve the unresolved sketches also.
Any suggestions and questions are welcome...
Well to start with I've used some random part which I made during my sophomore year..I dont remmeber any dimesion....
Now lets get started..
Select the front plane and go to sketch mode...
Now hold your "right mouse button" for a second and a menu pops-up..
Now when you hover your mouse over the part you'll notice many lines and axis can be felt.
Select the references you think will do your work.Close the pop-up menu.
One good thing is that you can easily locate centre of the arc easily...
this is my version of tweaking the part...
just extruded and done...
I never remembered any dimensions and still i was able to modify the part easily..