The name of a rank expression. Rank expression names must begin with a letter and can contain the following characters: a-z (lowercase),
            0-9, and _ (underscore). Uppercase letters and hyphens are not allowed. The names "body", "docid", and "text_relevance" are reserved and
            cannot be specified as field or rank expression names.
             
            
Constraints:
 Declaration Syntax
 Declaration Syntax| C# | 
public string RankName { get; set; }