A rank lexical relation is an association between lexical units which have different relative values based on some shared underlying semantic property.
Lexical units which are related by rank are likely to be functionally descriptive of the characteristics of an entity. They are likely to be lexical categorized as nouns.
A rank set has the structure of a scale.
Cruse, D. A. 1986.Lexical semantics. Cambridge, England: University Press.