For numeric types, contains the original text representation of the value. For booleans, "true" or "false". For strings, contains the default text contents (not escaped in any way). For bytes, contains the C escaped value. All bytes >= 128 are escaped. TODO(kenton): Base-64 encode?
For extensions, this is the name of the type being extended. It is resolved in the same manner as type_name.
JSON name of this field. The value is set by protocol compiler. If the user has set a "json_name" option on this field, that option's value will be used. Otherwise, it's deduced from the field's name by converting it to camelCase.
If set, gives the index of a oneof in the containing type's oneof_decl list. This field is a member of that oneof.
OptionaloptionsIf type_name is set, this need not be set. If both this and type_name are set, this must be one of TYPE_ENUM, TYPE_MESSAGE or TYPE_GROUP.
For message and enum types, this is the name of the type. If the name starts with a '.', it is fully-qualified. Otherwise, C++-like scoping rules are used to find the type (i.e. first the nested types within this message are searched, then within the parent, on up to the root namespace).
Describes a field within a message.