I am new to NetBSD
. I asked myself the same question when I decided to update my NetBSD for the first time. Apparently you need the "xsrc" sources if you're going to use X Window System. You put them in /usr, for example. After this you need to add the
-x switch to build.sh script to build X11 from chosen directory (default: /usr/xsrc).
$ ./build.sh -O ../obj -T ../tools -U -x distribution
These commands will compile NetBSD userland including "xsrc"