I find it confusing. It seems like Nibbana can refer to the cognitive proces freed from the six roots. That is something personal and of this world. On the other hand Nibbana is also refered to as not-of-this-world, or even unmade, unbecome and unconditioned. A cognitive proces is not unmade and not unconditioned and of this world. So, how can a cognitive proces ever refer to Nibbana?
I still find it a strange and unstatisfying idea that there have always been a certain number of lifestreams without any origin. They just have been always there. I have to digest this some way.