Error codes

[maivizhi_arunag]

As per the Apigee doc the default error code format is

{"code":" {ErrorCode} ","message":" {Error message} ","contexts":[]}

But when i execute my proxy with error ,The format is :

{
  "ErrorCode": "{ErrorCode}",
  "Error": "{Error message}"
}

and sometimes

{
  "fault": {
    "faultstring": "",
    "detail": {
      "errorcode": "xxx"} }}

would like to know when these error formats will occur and also when the default format will get executed?

Note : I dint add any FaultRules.

Correct me if i am wrong .Thanks!