I wouldn't consider those "internal". Most of them define public interfaces, no?

"used for the implementation and testing" then ?

"defining the interface across client libraries"?

Hmm not sure because as you said it is user facing and currently most of them are not used to communicate across client libraries given that they're used mostly/(only?) in rclcpp.

@ros2/team maybe some native speakers have some insights ?

They're definitely public and I would say their primary use is implementation of rcl concepts rather than testing, though they could be used for that too I guess.

"This repository contains a set of packages which themselves contain interface files (i.e. .msg and .srv files) used to implement client library concepts like parameters."

my goal here is to give a description of the content of the repository and not only describe the rcl_interfaces package, that's why I talked about testing (given that this repo also contains test_msgs).

I'm fine with "This repository contains a set of packages which themselves contain interface files (i.e. .msg and .srv files) used to implement client library concepts" I just wanted to clarify the scope

Oh, I was confused by the diff and I didn't know there were test messages here. In that case I agree, something like:

"This repository contains a set of packages which themselves contain interface files (i.e. .msg and .srv files) which are used both to implement client library concepts and in testing."

Or something like that.

I've updated it with approximately the verbiage of above.