It's a rolling release with minimal changes to packages from upstream, and generally the latest versions of available software in the repos. I guess you could go through and rebuild the whole system from source if you were determined to, but a quick look at the ABS wiki page doesn't make it seem like it's set up to make doing so all that easy. For other software not in the repos, the AUR makes it easy enough to build them from source, though there's often binary options available as well. The base install is pretty simple, so you can build upon it as you'd like if you really want to go wild on a minimal, highly customized system. Or you can go wild installing what you'd like and trying all the things.