cvsup collection question

[Tom Stockton]

Apologies for stupidity, I've read the documentation a million times and 
I've got a headache.

I've installed 4.7 release from an ftp server, I want to get the latest 
ports collection to match this release.  It looks like I want to specify 
'ports-all' in my cvsup file, however a note in the documentation says that

"The ports tree does not have any tag associated with it, it is always 
CURRENT."

I dont like the look of this...I'd like to specify the tag RELENG_4_7 but 
this note seems to make that redundant ?

Also what is the src-all collection ?  How does it differ from ports-all ?

Thanks for your time.

Tom