Should just be the messages actually exposed in dds_topics.yaml

[hamishwillee]

The folder appears to just be a dump of all messages in the PX4-Autopilot source tree. That means that if you're an end user you need to go back to the corresponding release and look at dds_topics.yaml to work out what you can actually use.

Much easier for developers if you say "this is the set of messages you can actually use in each release"