It's scientific notation, for representing really large numbers in a more compact way. Terragen works in meters and uses real world scales for almost everything. So for example the planet is the correct 6,370 kilometers, or 6,378,000 meters. 1e+06 is 1,000,000 (a 1 followed by 6 zeros), or a million meters, 1000 kilometers. This just establishes by default a pretty large maximum geometry export distance such that it will likely cover anything you need to export, without exporting literally *everything*, which would not only take a lot more time, but also be largely useless to most people. Anyway, you can set the max distance based on the general size of your scene. If you only need a few thousand meters of terrain from your camera, set it to say 5000. Leaving it at the default is fine too, you may just get more terrain exported than you actually need.
- Oshyan