What is enum DataSource.Link in flutter ferry?

I am looking at this code in the file operation_response.dart from ferry_exec 0.3.1 https://pub.dev/packages/ferry_exec

enum DataSource {
  /// A placeholder response source which can be used when waiting for another source
  None,

  /// Data originated from the client's [Link]
  Link,

  /// Data originated from the [Cache]
  Cache,

  /// Data originated from a user-provided [OperationRequest.optimisticResponse]
  Optimistic,
}

From the comment, I understand that None is a placeholder while waiting for data, Cache is getting the data from cache, and Optimistic is the data from the request, but I don’t understand what Link does. What does it mean and how do we use it?