A possible example in this example the Main extents is 5k by 5k, but I placed the wandering 2k extent to build the green volcanoes. Instead of building the volcanoes at 5k by 5k which 1. wasted RAM, 2. is performance intensive, 3. can result in a slightly different output due to differing renders the custom render extent would just build the volcanoes at 2k. This is similar to tiled builds exempt for the individual tiles dont exist on a grid and only a selection of devices uses the tiled builds.
