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As far as the maintenance period and renewals are concerned, they work the same for both options (and for most plugins for Atlassian products in general). When you purchase a license, it has comes with a one-year maintenance and support period. This means you will have access to all the versions of the plugin that will be released during this that year.
After the year ends, you can keep using the latest version available to you version perpetually, but you will not be able to upgrade to newer versions unless you purchase a renewal. A renewal will extend your maintenance period for another year and thus, will give you access to the versions that will be released during this yearthat 12-month period.
Please note, that the renewal extends the maintenance period for one year starting from the end of the previous maintenance period regardless of the time you purchase the renewal. For example, if the maintenance period expires in February 2013 and you purchase the renewal in May 2013, the maintenance will be extended until February 2014, not May 2014.
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