This substitution effectively duplicated enclosing RUN command for each file in the given path. Accepts wildcards.
In order to keep implementation simple, only a single such substitution per command is supported. It is possible to support multiple such substitution via cartesian product, if deemed necessary.
Use case for this is upcoming refactoring of C++ DR tests structure to avoid tests interfering with each other. Currently, tests are grouped by 100 per file, and have a RUN command per C++ Standard revision (for a total of 7 with the recent addition of c++2c mode). The plan is to rely heavily on split-file to invoke the compiler individually for each test, but avoid duplicating RUN commands for each of them. More details in Discourse thread.
It's completely unclear what "Accepts wildcards" means in this context. I wonder if an example might be appropriate.