What's the difference between the 'full feature set' and 'reduced feature set'?

The supported Magento versions (shown in the "Details" tab of extension listings) include all advertised extension features. 

Older Magento versions (shown under "Legacy Magento version support") have a reduced extension feature set. It is possible to run the extension on these Magento versions, but please be aware that not all advertised features will be included in the extension version you'll be installing. This is because many extension features require the functionality of more recent Magento versions to work their magic. 

Rest assured that if your store is upgraded to a later version of Magento in the future, you can still download and install the full feature version of the extension. You'll always have access to the extension, plus any new versions released during the 12 month free update period.