This annotation is a marker to indicate that a class is permitted and intended to be serialized as part of Node messaging.
Strictly speaking, it is critical to identifying that a class is intended to be deserialized by the node, to avoid a security compromise later when a vulnerability is discovered in the deserialisation of a class that just happens to be on the classpath, perhaps from a 3rd party library, as has been witnessed elsewhere.
It also makes it possible for a code reviewer to clearly identify the classes that can be passed on the wire.
Do NOT include AnnotationTarget.EXPRESSION as one of the @Target parameters, as this would allow any Lambda to be serialised. This would be a security hole.
TODO: As we approach a long term wire format, this annotation will only be permitted on classes that meet certain criteria.