CURRENT VS STABLE

FreeBSD has two development branches: FreeBSD-CURRENT and FreeBSD-STABLE.

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.

References:
Tracking a Development Branch;
Release Engineering.