The operation is not to copy but to snarf. It's called snarf because snarf is what it does. There is no design document.
Snarf is the term for Copy on the Plan9 operating system.
The cut operation deletes the selected text from the screen and puts it in the snarf buffer, snarf copies the selected text to the buffer without deleting it, paste replaces the selected text with the contents of the buffer.
The term has made its way into the Uxn ecosystem as the name for the clipboard buffer, programs will handle clipboard operations by writing to an external file called
.snarf. The applications Left and Orca will create snarf files.