Explanation
https://help.salesforce.com/articleView?id=mc_jb_schedule_an_audience.htm&type=5

Explanation
Einstein Copy Insights is the tool that should supply insights into the tone of subject lines. Einstein Copy Insights is a feature that uses natural language processing and machine learning to analyze your subject lines and provide suggestions for improvement. It also shows you the tone of your subject lines, such as positive, negative, neutral, or mixed, and how it affects your engagement metrics, such as open rate, click rate, etc.
Einstein Recommendations is a feature that uses artificial intelligence to provide personalized product or content recommendations for your subscribers based on their behavior and preferences. Einstein Messaging Insights is a feature that uses predictive intelligence to identify issues or opportunities with your email campaigns based on performance trends and anomalies.