F-Secure SAFE subscriptions are based on the number of devices you want to protect and the length of time you choose for your subscription; for example, a subscription can include up to 25 devices for a period of either 12 or 24 months. When you buy an F-Secure SAFE subscription, the payment for the billing period is seen as "credit". This "credit" goes down during the billing period until it is time to renew the subscription, and a new billing period starts over again.
If you choose to upgrade a current F-Secure SAFE subscription and add more devices (licenses) during an on-going billing period, the additional devices you purchase are added to the on-going billing period. The price of the additional licenses is calculated on a pro-rata basis depending on how much of the current subscription period is left. For example, if you have previously bought a 1-year subscription with 5 devices, and you want to upgrade to 7 devices in total, you pay for 2 additional devices (licenses) divided equally across the time left on the 1-year subscription. This amount is then added to your "credit".
When the subscription is up for renewal, the amount due to be paid includes the upgraded subscription with the new number of devices (licenses).