Some problems about Rebuild NetBSD

[linyin_cn%126.com@localhost]

I'm the newbie for NetBSD,I always using FreeBSD before.Now I'm insteresting in 
NetBSD.I wanna rebuild it via the source.I read the NetBSD Guide Book,I saw 
there are 3 version about NetBSD source code,there are NetBSD release 
branch,NetBSD stable branch and NetBSD current branch.
If I use Release Branch,what's the different with Stable Branch,is that like 
FreeBSD Standard source code,just fix some security problems?
 
Should I rebuild the system via source code? or only need rebuild kernel.I 
remember in the FreeBSD,when us update system we should rebuild world and then 
rebuild kernel.What about in the NetBSD?
 
By the way,does NetBSD have any special for the CPUTYPE,In the 
FreeBSD(/etc/make.conf 
http://www.freebsd.org/cgi/man.cgi?query=make.conf&sektion=5) there have 
CPUTYPE enabling optimisations more specific to my CPU,but I can't find any 
detail information about NetBSD.
 
I read the chatper about Crosscompiling NetBSD with 
build.sh(http://www.netbsd.org/docs/guide/en/chap-build.html), In that 
chapter,I can't understand what is userland use for,is it important? And I also 
can't understand with Static Builds and Toolchais.I'm not english native 
speaker.Who can explain to me more easy to understand.
 
Regards