Absinthe.Federation.Notation.key_fields

You're seeing just the macro key_fields, go back to Absinthe.Federation.Notation module for more information.
Link to this macro

key_fields(fields)

View Source (macro)

Adds a @key directive to the type which indicates a combination of fields that can be used to uniquely identify and fetch an object or interface. This allows the type to be extended by other services. A string rather than atom is used here to support composite keys e.g. id organization { id }

Example

object :user do
  key_fields("id")
  field :id, non_null(:id)
end

SDL Output

type User @key(fields: "id") {
  id: ID!
}