eintr: b for interactive
‘b’ in an Interactive Expression
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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’. 
The Definition of ‘append-to-buffer’
append-to-buffer.) The lower-case ‘b’ tells the Lisp interpreter that
the argument for ‘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.
The new and simplified code generates a list for ‘interactive’. It
uses the ‘barf-if-buffer-read-only’ and ‘read-buffer’ functions with
which we are already familiar and the ‘progn’ special form with which we
are not. (It will be described later.)
Info Catalog
eintr: Read-only buffer
eintr: insert-buffer interactive