Aktiia uses a new method to estimate your blood pressure: to avoid the discomfort and inconvenience of inflating a cuff around your arm multiple times per day, the Aktiia bracelet uses optical sensors that indirectly estimate your relative blood pressure changes. However, to obtain absolute blood pressure readings in units of mmHg, Aktiia still requires what we call an "initialization" that needs to be done once every month and involves using a traditional blood pressure cuff during a short session.
You can think of it as setting the right time when you buy a new watch. If you don't perform this step, your watch will still show you that time is passing, but it won't indicate the same time as your friend's watch. The same applies to the Aktiia monitor: if the initialization is not performed, the bracelet will still measure the fluctuations in blood pressure. Still, it will not show a value in mmHg that can be compared with another blood pressure monitor. Similarly, as you need to periodically readjust your watch to ensure it is still accurate, a new initialization is required every month to ensure the accuracy of the Aktiia bracelet.