Normally the Comments associated with an invoice are treated as a single block of text and are printed in whatever font is defined for the "detail" area of the document.
When System options 149 and/or 150 are turned on the comment text is broken into as many as five different comment "types": System, System (Loading Sheet only), Customer, Invoice and Corrosive Hazard. The "Comment Fonts" tab has been added to System Preferences to allow fonts to be defined for each of these five comment types.