RELENG_X gets you FreebSD X-STABLE, the continuous development branch that each release is taken from.

RELENG_X_Y gets you FreeBSD X.Y-RELEASE, plus any security fixes that are available for FreeBSD X.Y-RELEASE

RELENG_X_Y_Z_RELEASE gets you FreeBSD X.Y.Z-RELEASE, the exact same bits that go on the release CDs.

Fill in X, Y, and Z as needed.