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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 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 year.
After the year ends, you can keep using the latest 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 year.
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