The carb stone is another dual purpose device! It can be used to oxygenate your wort when hooked up to an oxygen source with both the FLEX and FLEX+. It can also be used to add pressure for transfers, purging oxygen, carbonating, etc when used with the FLEX+ which is rated for 15 psi. If reducing your carbonating time from upwards of a week down to less than 24 hrs is appealing then the FLEX+ and a carb stone bundle might be your best friend! It also has a protective stainless housing which prevents damaging the very brittle 'stone'.
The carb stone attaches to a 1.5" TC. It will attach directly to the FLEX+ butterfly valve. The FLEX will need an upgraded 1.5" butterfly valve, a 1/2" NPT to TC adapter or can be attached to the secondary port.