What does the ProtoInclude attribute mean (in protobuf-net)

In the ProtoBuf-Net implementation, what does the ProtoInclude attribute mean, and what does it do?

An example would be appreciated.

I saw it in this post and I'm not sure what it does. The example was:

[Serializable,
 ProtoContract,
 ProtoInclude(50, typeof(BeginRequest))]
abstract internal class BaseMessage
{
  [ProtoMember(1)]
  abstract public UInt16 messageType { get; }
}

[Serializable,
 ProtoContract]
internal class BeginRequest : BaseMessage
{
    [ProtoMember(1)]
    public override UInt16 messageType
    {
        get { return 1; }
    }
}

Also, is there a way to generate such inheritance using the protogen tool?