The flashbacks (SPOILER)

Both.
By the end of the film it is revealed that, through contact and communication with the aliens, the protagonist (Amy Adams) has gained a new perspective of time, that her consciousness has actually been altered, and the film is told in the same fashion. We, the audience, jump through time the same way she does, forwards and back, at times knowing and not knowing what is to come. Or maybe I should say it’s the narrative that jumps, and we are tagging along for the ride.
This is a film that rewards every rewatch.