Checkmarking "Enable Templated Email" allows you to specify the entire text of the post-checkout thank-you email. Use this option if you want to completely override our standard thank-you email format and instead substitute your own, complete message from start to finish. Note that in this case, you will need to type special [%codes%] among your regular text, to include order-/item-specific information, such as the download link. Consult our help-page reference to these codes here:
However, even without marking the Enable Templated Email checkbox, you can still simply add your own custom text to our standard thank-you/download-link emails, by typing whatever additional text you desire into the Email Message textbox provided on the first Add/Edit config screen for each product in your Seller Admin.