main.cf and cyrus-imapd (cyrdeliver)

Hi,

I'm using postifx and cyrus-imapd.
In the file main.cf there is two lines :

cyrus unix - n n - - pipe
flags=R user=cyrus argv=/usr/sbin/cyrdeliver -e -m ${extension} ${user}

What's the utility of "flags=R" ? I can't find any doc about this
setting ...

Another little question about cyrus : when I man cyrdeliver, there is two configurations parameters :
-r address
Insert a Return-Path: header containing
-f address
Insert a Return-Path: header containing

What's the differences between those 2 parameters ? What's the utility to
insert a return-path ?

In advance thanks and sorry for my @#$! english ;\