Subject lines are generated by the AI based on the data of the individual user. The default setting is such that the subject line is the title of the top recommended article for the user.
Here is an example of the recommend article generating the subject line for a subscriber.
While it is recommended to use the AI generated subject line for optimal engagement, it is possible to edit the subject line to be the same for all subscribers. Select User → Edit Configurations in the upper right corner of the dashboard. Once you are on Configurations, click add then select email_subject for the key and then add the text that you would like to appear for the value and click the save button under edit.