Nuzūl describes a chain having more intervening narrators than a competing route. The route is called nāzil in contrast to one that is ʿālī.
Example: One route has seven links to the Prophet ﷺ and another nine. The former is elevated and the latter lower by count. Comparing routes to a particular imam rather than the Prophet produces relative elevation and its corresponding descent.
Corresponding categories: Each kind of elevation has a comparative type of descent.
Nuzūl concerns isnād distance, not automatic rejection or weakness of a report.