CURRENT VS STABLE
FreeBSD has two development branches:
FreeBSD-CURRENT is the active development branch of
FreeBSD, and all new development work, such as adding new feature, will occur in
FreeBSD-CURRENT branch firstly.
FreeBSD-STABLE is span off from
FreeBSD-CURRENT, and for production version. It mainly accepts bug and security fixes.
You should notice, branches are not points but constant streams of development. And the
FreeBSD RELEASE version is a point, and it is just a snapshot of
FreeBSD-STABLE branch at some time.