Richard Stallman <rms@gnu.org> writes:

> What does this feature do?
> Where is the text to describe it?

The code implements:

    (define-key ctl-x-r-map "N" 'number-rectangle)

Where C-h f number-rectangle is described as:

    Insert numbers in front of lines in rectangle BEG END.

    START-AT specifices the first number to start at.
    FORMAT is format string where %i denotes inserted number.

    If prefix arg NO-ZERO-PADDING is non-nil, do not padd numbers

Demonstration, initial data:

     Item one
     Item two
     Item three

Put cursor at first line's character, draw region up till last line's
first character. Call M-x number-rectangle:

     First number [1] (default 1): <RET>
     Format: %i <RET>

Result:

     1 Item one
     2 Item two
     3 Item three

It the region would have been longer, say over 10 lines, the padding
would have taken in effect:

     01 Item one
     02 Item two
     ...

Jari

Here is slightly updated patch which corrects the name of the function
to `number-rectangle'.

2009-03-22  Jari Aalto  <jari.aalto@cante.net>

        * rect.el (number-rectangle): New user function.
        (rectangle-number-line-callback): New function.
        (rectangle-number-line-format-history): New variable.
        (rectangle-number-line-counter): New variable.
        (define-key ctl-x-r-map "N" 'number-rectangle): New
        user key binding. The basis of the rectangle numbering code was
        lifted from gse-number-rect.el, orignally developed by Scott Evans
        <gse@antisleep.com>. Used by permission (GPL).