Indicates a field should be ignored when its declaring class is registered unless the
context has
a value set for the specified key. This can be useful when a field must be serialized for one purpose, but not for another.
Eg, a class for a networked application could have a field that should not be serialized and sent to clients, but should be
serialized when stored on the server. If a field has multiple of this annotation, then the field is serialized if at least
one of the keys is present in the context.