planet creates single-sided planes, at a variety of resolutions. The tool works exactly the same as the trueSpace plane button, but creates less geometry.
Note also that the current versions of trueSpace don't handle non-solid polyhedra in some tools at all (quad divide and boolean being two of the main examples), and single-sided planes are not solid. The excellent ThermoClay plugin from Quantum Impulse does handle non-solid polyhedra, and using planet in conjunction with that tool makes it very easy to create objects such as drapes, cloth etc. with half the usual geometry.
planet is fully supported under all versions of trueSpace