It is an element because it bears characteristics and so on, as explained by Buddhaghosa.Alex123 wrote: IMHO, Nibbāna should not be classified as any of 12 sphere or 18 elements.
It is an ultimate reality and therefore must be either nama or rupa because those are the only two types ultimate realities. It is not rupa because it is not physical. It is nama but a kind that does not experience anything itself. It does not arise or fall and can be experienced by nama when the conditions are correct. Experience of it uproots defilements and so on.