Refundable Rules

Tried searching but was unsuccessful --

If I have a ticket which has both a refundable segment and a non-refundable segment, and I now need to cancel the trip, how does the refund work?

Entire PNR refundable?
Entire PNR non-refundable?
Refundable segments of PNR refundable and non-refundable segments of PNR not?