Flatsim (which stands for “flatworld simulation”) is a lightweight application that allows you to run most of the Isaac Navigation Stack in a 2D simulation. The Flatsim application simulates a laser range scanner by casting rays in a given occupancy grid map. This simulation is extremely fast compared to a more costly 3D ray scan in a full 3D environment. It is thus a quick and highly performant way to test the navigation stack.
The Flatsim application cannot provide the same accuracy as a full 3D simulation of the same scenario, especially for accurate simulation of reflections or transparent objects.