An NFR was once a Story… an Enabler (a type of a story) to ‘enable’ — like a needed piece of the puzzle to complete the picture — a certain functionality; then once the NFR is built, it becomes a constraint to the backlog.
SAFe drew the NFR as a gray oblong shaped figure under the backlog… this is to indicate that NFR is a constraint to the backlog (i.e. program backlog, team backlog, solution backlog and epic backlog) — see picture below)… lots of people are not aware of this.
Here is another visual from SAFe on NFR as constraint to the backlog — from the SAFe requirements model:
The bigger picture of the SAFe requirement model:
If you look at NFR as once an Angel … then later becomes the Devil … you, as an architect (usually, architects bring forth NFRs), have to think twice of the NFR item that you are about to build… it will constrain all backlog items that will use it.