Ticket email notifications have a standard subject: "Your ticket for (event title)" if you wish to change the subject, you definitely can.
To change the subject of your Email notification, you can go to the event, then on the event dashboard, click onEvent Settings
Then in the Event Settings, click on the Notifications tab
Scroll down on Templates ,and you will see the ticket types (Ticket notification, Multi / Group ticket email, and Attendee messages)
Click on Edit, then choose Customize email template to change the subject, then hit Save
Make sure to choose the Customize email template, or you won't be able to change the subject.