When exporting data to email, you can edit the email subject line to suit your preferences.
You are also able to add the dynamic order number and date/time to the email subject.

Add the order number to the email using the placeholder {{source.first_order_number}}. The placeholder can be added between any words.
You can also add the date and time to the email subject by pasting this code in the Email subject field.
You can choose to add before, after, or in between the subject by placing the code accordingly.
The date-time format follows the file date format.