A rich set of characteristics that define the behaviour of the Data Distribution Service (DDS) systems (such as reliability, liveliness, durability, etc.). QoS Policies control the flow of the data through the system. The Topic, Data Reader, Data Writer, Publisher and Subscriber all have QoS polices. The QoS policies of Publisher, DataWriter, and Topic control the data on the sending side. QoS policies of Subscriber, Data Reader, and Topic control the data on the receiving side. These must be of a compatible type for successful communication, i.e. a Publisher/DW with a BEST_EFFORT QoS cannot send samples for a Topic with a RELIABLE QoS as it could degrade the Topic. However, a RELIABLE Publisher/DW can send samples for a BEST_EFFORT Topic.
Source: OpenSplice Glossary