Specifically denotes a field as optional. While all fields in protocol buffers are optional, this may be specified for emphasis if appropriate.
REQUIRED
2
Denotes a field as required. This indicates that the field must be provided as part of the request, and failure to do so will cause an error (usually INVALID_ARGUMENT).
OUTPUT_ONLY
3
Denotes a field as output only. This indicates that the field is provided in responses, but including the field in a request does nothing (the server must ignore it and must not throw an error as a result of the field's presence).
INPUT_ONLY
4
Denotes a field as input only. This indicates that the field is provided in requests, and the corresponding field is not included in output.
IMMUTABLE
5
Denotes a field as immutable. This indicates that the field may be set once in a request to create a resource, but may not be changed thereafter.
