I believe that in this context N.C. would imply that no chord is being played really and the (F5) would suggest that the harmony would have been evolving around the F Power Chord.
Per example, you could have Dm7 G7 N.C.(Cmaj7) C#m7b5 or something. This would sort of mean a break in the underlying chords BUT give you a hint on how to improvise there and is simply a matter of arranging the song.
(I think, that's how I would've used it I'd like som C or C (confirmation or correction, yeah I just made that up
)).