That's referring to the sanding drum. If you drop to the bottom of the manual you'll find the parts list; the first part (coincidentally) is the belt drive motor and is listed as a "Gear Motor ... 90 Volt DC".
As an aside 90V DC motors are a common voltage for motors driven from residential circuits (as opposed to a battery) and similarly you'll find that there is a great availability of 90V DC motor speed controllers.