Class Preference (1.21.0) 

Preference(value)

This field indicates whether this backend should be fully utilized before sending traffic to backends with default preference. The possible values are: - PREFERRED: Backends with this preference level will be filled up to their capacity limits first, based on RTT. - DEFAULT: If preferred backends don't have enough capacity, backends in this layer would be used and traffic would be assigned based on the load balancing algorithm you use. This is the default

Enums
Name Description
UNDEFINED_PREFERENCE A value indicating that the enum field is not set.
DEFAULT No preference.
PREFERENCE_UNSPECIFIED If preference is unspecified, we set it to the DEFAULT value
PREFERRED Traffic will be sent to this backend first.