A road can either be adopted, unadopted, or private. That's it.

Take my local council. Their website states: "The majority of the roads in Southampton are public highways maintainable at the public expense. These roads are cleaned, lit and repaired by Southampton City Council and are referred to as adopted roads. The remaining roads are private streets [unadopted roads] or private roads. Maintenance of these roads is the responsibility of the adjacent landowners whose property fronts onto the private streets or private roads. The public has the right to use private streets even though they are not maintained by the council."

Quite clear why this road isn't in Norfolk's list of streets. Clearly states "The List of Streets only indicates highways that are maintainable at public expense and does not include streets that are privately maintained" - so unadopted roads aren't included.