The Robot Operating System 2 (ROS 2) is a set of software libraries and tools that help you build robot applications e.g, hardware driver, simulation, build environments.
The Data Distribution Service (DDS) is more a plain middleware (aka RMW) layer allowing to define use case specific messages. The ROS2 message (s/ types) can be found here.
Besides the robotic specifc messages a well defined threat model gives further guidance about potential attacks.
What else? It has a great community, sharing papers such as “how to implement realtime systems“. IMHO the community is th emost important factor.