is the end-of-life calendar for the freebsd releases.
I don't think FreeBSD has the concept of "back-porting" security patches to -RELEASE.
It does. This is the security branch associated with the release. This is explained in the link above. For 7.2-RELEASE, the
associated security branch is RELENG_7_2. So, if you install 7.2-RELEASE and ran
freebsd-update fetch install
you would end up with the latest build from RELENG_7_2, which today is 7.2-RELEASE-p2.