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): Specifies what filed the data will be read from (or what view column)

json: specifies what key name it will have in the resulting JSON data

type: specifies the data type of the field/column, for multivalue fileds/columns use ARRAY_OF_x