Upgrade a single package from a target release

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http://unix.stackexchange.com – In Debian Squeeze I have installed a packageX from debian-backports. Q1: When I apt-get dist-upgrade or aptitude full-upgrade, what release is checked for updates for packageX? debian-backports or debian-stable? Q2: Can I use a command to update a single package from a specific release? For example is aptitude -t full-upgrade packageX a valid command? I tried aptitude -st full-upgrade packageX but the output was not helpful. Q3: For completeness purposes, what are the equivalent answers for RPM based distros? (HowTos)