The complex endorsement applies to all airplane classes, including single and multi-engine as well as land and sea.

It allows the pilot to act as pilot in command (PIC) of seaplanes with flaps and a controllable-pitch propeller.
In the case of airplanes, pilots with complex endorsement can operate aircraft with retractable landing gear, flaps, and a controllable-pitch propeller.  It should be noted that the presence of flaps in an airplane or seaplane does not require a pilot to have the complex endorsement in order to fly that aircraft.

Unlike a pilot certificate or rating, the endorsement has no corresponding check ride or minimum number of flight or ground hours that must be completed. Training is completed at a pace that is comfortable for the student and to accommodate the amount of training required. Plus, the duration and the cost of training will depend heavily on the pilot’s proficiency level.