In many cases, when sending emails, you like to include some details from the invoice in the accompanying text. To make this possible, you can add variables to the email message or the subject line. These variables will automatically be converted to the correct value based on the sent document when the email is sent.
Some examples of available variables: %client-name%, %document-number%, %due-date%, %total-remaining%, %structured-message%
An overview of all available variables can be found here: Variables