[Koha] sort by Popularity

Can anyone explain the basis for the sort by "Popularity" in the Koha OPAC? Is it based on checkout frequency? # of copies owned? User ratings? ...? Thanks! Lynda. -------------------------------- Lynda Howell Systems Librarian / Technologist Union Institute & University 62 Ridge Street, Suite 2 Montpelier, Vermont 05602 (802) 828-8618 lynda.howell@myunion.edu