For a document type, you can have more than one default report set for printing. When sending a document by email, the name of the second report — i.e., the one that was added last — is inserted into the subject line of the message.
In this example, the text "Invoice" is inserted into the email subject, based on the name of the Invoice report.
💡 If you need different text in the subject line, rename the report that is listed second for the document type.