What number is assigned to type field during Protobuf serialization?

Libraries
1

What number is assigned to type field during polymorphic Protobuf serialization and how to control it?

There is a special annotation @ProtoNumber for customclass fields, but what number is used for automatic “type” field in case of open Polymorphic serialization?

2

As I revers engineered polymorphic properties stored in wrappers with fields:
string type = 1
bytes value = 2

3

It is the standard way to serialize map’s entry.