The next argument in the interactive expression starts with a lower
case b. (This is different from the code for
append-to-buffer, which uses an upper-case B.
See The Definition of
The lower-case b tells the Lisp interpreter that the argument
insert-buffer should be an existing buffer or else its
name. (The upper-case B option provides for the possibility
that the buffer does not exist.) Emacs will prompt you for the name
of the buffer, offering you a default buffer, with name completion
enabled. If the buffer does not exist, you receive a message that
says “No match”; your terminal may beep at you as well.