Proper use of Bearing Source

Greetings Fellow Falcon50 EX-ers!!

I am looking for good examples of where one might set the Bearing source different from the Nav source. I am currently setting both as the "GPS " Or  "VOR1" ( For ILS approaches). I  could use some help understanding the difference between the two.

 In G1000 cockpits for example( Phenom300) ,  I can set NAV1 and NAV 2  and see the "Green single arrow" or "Green double arrow" on the PFD  and use it as guidance from nearby VORs and STiILL be flying in GPS mode ( magenta)

 So what is the Bearing  source in FA50ex and how do i use it properly?

 best regards

Hari