Every day at noon we're getting all current VAT rates from https://vatinfo.eu, which means we have all vat rates accurate. (We assume they don't change this during the day) So whenthe vat rates are not the same as in your database, the component will update them dusring the nightly update.
How to enable it?
The only thing you have to do is navigate to the Configuration of RD-Subscriptions and turn it on. It will check on the next nightly cron if there is a change and update the rates for you automatically.
How can I detect changes?
When have logging enabed it will write entirs in your log. You can find it in the log folder of your Joomla inatllation.