thr threads considered 'bobbin-thread' are finer than regular sewing thread...usually the aurafil threads are either 40 or 50 wt...bobbin thread is 100 wt. (the higher the number the thinner/finer the thread...12 wt is like using string)
i keep the 2 separate, bobbin thread for the bobbin, 40-50 wt thread for the top...i did run out of white thread one time and used my 'sure-fine' bottom wt thread...it broke every 2-3 inches and made me a crazy person till i stopped and went to the store... i also have a designer one...my machine is very (picky) about threads...i have had some very expensive threads just not work...so, i have to say...use what works for you. if bottom line works on the top...good for you!