Sort-of, but not necessarily. It's a lot easier to get the format right if you know there's an authenticator and you know what length it is and it's totally sep

Sort-of, but not necessarily. It's a lot easier to get the format right if you know there's an authenticator and you know what length it is and it's totally separate from whatever is coming next -- so you still want clear, out-of-band signaling for the real data. Once you have that format, you're right that the exact serialization doesn't matter. The extreme example of this is XML DSIG and XML canonicalization in general.