Enum DurabilityQosPolicyKind_t, different kinds of durability for DurabilityQosPolicy.
The Service does not need to keep any samples of data-instances on behalf of any DataReader that is not known by the DataWriter at the time the instance is written. In other words the Service will only attempt to provide the data to existing subscribers
For TRANSIENT, the service is only required to keep the data in memory and not in permanent storage; but the data is not tied to the lifecycle of the DataWriter and will, in general, survive it.
Data is kept on permanent storage, so that they can outlive a system session.