cvsup ports question Freebsd 5.2

[jens]

Hi folks,
I have build the ports tree using the default ports-sup file tag 
tag=.

I want to understand why my ports tree is wiped off when i change the tag 
value to 
RELENG_5_2_0_RELEASE
or 
RELENG_5
or 
RELENG_5_2

in order to get the ports updating. I followed the instructions of the bsd  
manual assuming to stick to the latest stable version/branch.

Exerpt of the ports sup_file

# IMPORTANT: Change the next line to use one of the CVSup mirror sites
# listed at http://www.freebsd.org/doc/handbook/mirrors.html.
*default host=cvsup2.FreeBSD.org
*default base=/usr
*default prefix=/usr
*default release=cvs tag=RELENG_5_2_0_RELEASE
*default delete use-rel-suffix


does that mean there is no upgrade of the ports. I assume not because the 
whole tree is wiped off. 


Thanks.