> Rewrite links:  internal / external
> are not explained in the help file.

Yes.. argh I'll have to do it one day. Take note that 
this feature is reserved 'For experts only' (because 
this feature is not very useful for common users), but 
let's look at it:

Rewrite links:  X / Y

Where X is used for all links DOWNLOADED (that is, 
browseable locally on your hard disk)

and

Y is for all other links (not downloaded), which can't 
be accessed locally (external links, or links 
forbidden by rule).

X and Y can be either :

Relative URI (link WITHOUT <http://www.foo.com>..' and 
which may contains ../): for example, '../foo.gif' - 
the default form used for a locally downloaded file

Absolute URL (link with <http://www.foo.com>..'): for 
example, <http://www.foo.com/images/foo.gif> - the 
form used by default for external or not downloaded 
links

Not very common:

Absolute URI (link WITHOUT <http://www.foo.com>..' and 
which starts with a leading '/'): for 
example, '/images/foo.gif' - form not very useful for 
most people - ignore

Original URL (as written by the html page author): The 
URL, as it was on the original HTML page - useful NOT 
to change ANYTHING for links on downloaded HTML pages: 
this allow to copy a perfect mirror of a website, EVEN 
if it may NOT work peoperly on a local filesystem (due 
to various absolute links or absolute URL links inside)

Therefore,
'Relative URL / Absolute URL'
is generally the only useful option for most (99%) 
users.