The Data-Distribution Service (DDS) relies on the use of QoS. A QoS (Quality of Service) is a set of characteristics that controls some aspect of the behavior of the DDS Service. QoS is comprised of individual QoS policies (objects of type deriving from QosPolicy).

The QosPolicy objects that need to be set in a compatible manner between the publisher and subscriber ends are indicated by the setting of the "RxO" (Requested/Offered) property:

An "RxO" setting of "Yes" indicates that the policy can be set both at the publishing and subscribing ends and the values must be set in a compatible manner. In this case the compatible values are explicitly defined.
An "RxO" setting of "No" indicates that the policy can be set both at the publishing and subscribing ends but the two settings are independent. That is, all combinations of values are compatible.
An "RxO" setting of "N/A" indicates that the policy can only be specified at either the publishing or the subscribing end, but not at both ends. So compatibility does not apply.

The "Changeable" property determines whether the QosPolicy can be changed after the Entity is enabled. In other words, a policy with "Changeable" setting of "NO" is considered "immutable" and can only be specified either at Entity creation time or else prior to calling the enable() operation.

Default QoS values