I would like to understand in what coordinate system an .stl is exported when exporting a segmentation. From what I understand from: Coordinate systems - Slicer Wiki is that it would be exported in anatomical coordinate system, where you can then choose LPS or RAS.
Is there a way then to export in world coordinates, or to convert the exported points to world coordinates somehow?
RAS are the world coordinates in Slicer, so exporting in world coordinates is the same as exporting in RAS. However, I would recommend LPS for file export for compatibility with other software.
If you applied a transform on the model then you need to harden it on the model before exporting.