b0298358_h_API___OCLC_Developer_Network__Description
[Parameter] sortKeys [Required?] optional [Possible Values] relevance, Title, Author, Date, Library Count, or Score default sort key is relevance; relevance is only descending. default sort direction is "ascending"; to switch to descending add ",,0" to the sortKey. Separate combined sortKeys with a space; multiple values only works if frbrGrouping=off [Description] 
specifies how the result is sorted
Bibliographic Resource - WorldCat Search API | OCLC Developer Network