Subscribe is a payment which buys use of the tool for 30 calendar days and all updates, if any, during that period. To subscribe you need a Paypal account. Paypal will debit you every month. If you no longer wish to subscribe you need to cancel the subscription in your Paypal account.
Purchase is an one-time payment which buys use of the tool and all updates, if any, for either 30 days or for an undefined period of time as long as the tool can be offered. To purchase you need a credit card or a Paypal account.
After you have paid, log in and out of the platform to activate your tool.