Mac OS: 7.5.3 Command-Turn On I'm not a mac user anymore. But I used to be one when I was a kid.
My father had a Macintosh Performa 6200 running MacOS 7.5.3.

On said MacOS you could press command + the turn on key on the keyboard, and a white rectangle with a textcursor would appear.

Does anyone know what type of commands (and in what language) this box expected?

I googled around and found no mention of this textbox. Until this day I have not the slightest idea what that thing was.
That was the Debugger window that was built into the Mac's ROM. I never learned what language worked in it.