aliasable_field is used to indicate that a field is aliasable.

domain is used to associate the messages in a file with a domain package.

generate_sdk_option is a directive used when generating the SDK to indicate that an option should be created for the field.

mask_mapping is an option which tags a field with the expected field mask used by a companion proto if applied on the field this option is for.

subtype is used for oneOf attribute subtypes, to identify that we only need to parse attributes of oneOf once and not for each option when generating the API This will serve as a JSON key, used to transform that into the key the protobuf expects optional string subtype = 85555;.

subtype_source_id is used to indicate that a field provided an id that can be used to determine the subtype of a message.