The OpenStudio gbxml forward translator at least writes both.

• The RectangularGeometry defines a kind of an area-equivalent bounding surface: this represents the overall geometry of the surface, by defining an equivalent rectangular surface with the same area, a starting point, and a tilt,
• The PlanarGeometry element has a PolyLoop element with all points / vertices describing the surface as it is actually in 3D space.

https://github.com/NREL/OpenStudio/blob/d06c9047fd348e7edf52090486279f210fe947f9/src/gbxml/ForwardTranslator.cpp#L891-L978

Couple of other sources that discuss this:

• https://github.com/ladybug-tools/spider/blob/23bb9569dae07fecb076daf0fa21e6c4cc11fcc7/gbxml-user-guide/surface.md#planar-geometry
• https://www.mdpi.com/802174
• https://www.aivc.org/sites/default/files/p363_final.pdf