The built-in letters are for generic usage. Custom attendance letters are generally handled through mail merge reports. They use a locally saved Word document for school-specific formatting, wording, letterhead, etc.
For a quick how-to, please see: Using Mail Merge : SchoolWise SIS Knowledge Base
If you are wanting to track when truancy letters are sent, the SARB letters would be used for that.
All the reports in 'Attendance - SARB' use the student SARB Action to track when letters are printed.
A corresponding SARB Action can be automatically added when you run the report.
For assistance in creating a custom Word document that can be used in the chosen mail merge report, please contact James@SchoolWise.com