When creating an email template, one can also use parameters.  

Those are data elements from the entity itself (SP/CS/SO/Invoice etc) that can auto populate a specific field in the template, accordingly.

Parameters can be used in the email template subject and body.

When creating an email template:

1. Make sure to choose the correct entity type

2. In order to use parameters in the subject, search for the relevant parameter, choose it.  It will automatically show in the body of the email.  It just needs to be copy-pasted to the subject line:

3. In order to use a parameter in the body of the message, just choose the relevant parameter from the list and place it in the required location.