return to the Quality of Service (QoS) Parameters

The purpose of USER_DATA QoS is to allow the application to attach additional information to the created Entity objects such that when a remote application discovers their existence it can access that information and use it for its own purposes. One possible use of this QoS is to attach security credentials or some other information that can be used by the remote application to authenticate the source. In combination with operations such as ignore_participant, ignore_publication, ignore_subscription, and ignore_topic these QoS can assist an application to define and enforce its own security policies. The use of this QoS is not limited to security, rather it offers a simple, yet flexible extensibility mechanism.

Source: DDS 1.4 Spec