Message definition namespaces

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According to the ROS Messages page:

Message types use standard ROS naming conventions: the name of the package + / + name of the .msg file. For example, std_msgs/msg/String.msg has the message type std_msgs/String.

In your example, it could make sense to make a generic vehicle message say pkgname/GenericVehicle.msg and then create vehicle specific messages for vehicle1 and vehicle2. These specific messages could then have a field entry that is the pkgname/GenericVehicle.msg. In this way you could compose messages that hierarchically depend on each other such that, when used in a client library, could become something like:

msg = pkgname.msg.Vehicle2()
msg.status.electrical.core_electrical_value = 1.0