The ID of the transfer being accessed

Used for pagination to retrieve the next set of results. A next_start_key is generated when a limit argument has been provided and more results are available for retrieval.

A comma separated list of properties to include in the output.
For example, if objects of the form

{
  attrA: ...,
  attrB: {
    subB1: ...
    subB2: ...
  }
  attrC: ...
}

are usually returned, specifiying attrA,attrB.subB1 will
instead return objects of the form

{
  attrA: ...,
  attrB: {
    subB1: ...
  }
}

Note that there should not be spaces around commas.