Every bonus will have specific requirements and conditions on how to be played. These conditions will be mentioned in the notification you receive once a bonus is added to your account.
Your bonus account will be updated with the last amount accepted. For example, if you have 5 euros in the bonus account and you accept a new bonus of 10€, your bonus amount after acceptance will be 10€. However, in the event that you already have a bonus amount of 10€ and a new bonus of 5€ should be credited to you, the bonus will not be added.