Essentially selecting and operating on text in the same buffer. Saw it in Russ Cox’s demonstration of Acme.
Basically any function/procedure that gets exported from a module can be used either on the REPL, or from them mouse, depending on its signature.
Quite powerful concept for those that like to take advantage of GUI based OSes.
Powershell is the only that comes close to it. Maybe fish as well, but never used it.