Attaching a document to a service makes it possible to print a specific letter template when chart a particular service. i.e print an educational page on "What to expect when having an extraction".
Make sure the letter template already exists (See: Creating, Importing & Editing Letter Templates) then select the letter from the edit service screen, then choose whether it prompts to print when being charted, double clicked (disabled) or ticked as completed.
Example of the prompt appearing after charting the service with the document set to "Pop up when planned". Name the document and click "ok" to print.