Astarte.Device.CredentialStorage.save

You're seeing just the callback save, go back to Astarte.Device.CredentialStorage module for more information.
Link to this callback

save(key, value, state)

Specs

save(key :: credential(), value :: String.t(), state :: term()) ::
  {:ok, new_state :: term()} | {:error, reason :: term()}

Save the specified credential to the credential storage.