The opt-in notification rate refers to the percentage of individuals who have actively chosen to receive notifications from a particular source or platform. This rate is a measure of the effectiveness of opt-in marketing strategies, as it indicates the level of interest and engagement from the target audience. A higher opt-in notification rate suggests a strong and engaged audience, while a lower rate may indicate a need for improvement in the messaging or targeting of notifications. This metric is commonly used in digital marketing to track the success of opt-in campaigns and to optimize future notification strategies.