mapJson is router parameter, which defines what notes field or view column will be returned in resulting json.
Syntax
mapJson 'notesFieldName/columnProgrammaticName', json:'jsonFieldName', type: 'STRING|INTEGER|DOUBLE|DATETIME|ARRAY_OF_STRING|ARRAY_OF_INTEGER|ARRAY_OF_DOUBLE|ARRAY_OF_DATETIME', isformula: 'true/false', formula: '', readonly: 'true/false', writeonly: true/false
notesFieldName (or columnProgrammaticName for VIEWENTRIES* strategies)