You've come across one of many areas where the FAA introduced unnecessary confusion into the Sport Pilot rules by using nonstandard terminology. For all other levels of pilot certificate, the category is "airplane," "rotorcraft," "glider," etc. Within these categories are classes of aircraft such as "single-engine land," "single-engine sea," "helicopter," "gyroplane," etc. All existing pilots are familiar with this structure. For sport pilot privileges, the FAA decided that "airplane," "powered parachute" and "weight-shift control" would be called classes of aircraft instead of categories. Within these classes there are "sets" of aircraft that break the classes down into broad groupings of aircraft with similar characteristics, such as tricycle gear, tailwheel gear, seaplanes, etc. This is a new way to talk about these aircraft groupings, totally different than what existing pilots are used to.