Imagine that only some of the sources contained a Fader node. With the current database types ConsumerTRootCreateAsync would throw an exception for the first source that does not contain a Fader node. To avoid this, elements can be declared optional:

private sealed class OptionalFaderSource : FieldNode<OptionalFaderSource>
{
    [Element(IsOptional = true)]
    internal Fader Fader { get; private set; }

    [Element(Identifier = "DSP")]
    internal Dsp Dsp { get; private set; }
}

Proceed to Page 15: Nullable Parameters.