Since no one has answered your question, I will do my best.
As far as I am aware, NU is NOT a subscription based product. Once you've bought it it's yours. When new versions are released you normally need to buy again. I say normally because I have seen where Symantec have released a new version and given those that had recently purchased the previous version the chance to upgrade either free or at a reduced price. I can't remember which now, sorry.
I hope that answers your question.