GeoJSONL is a variation on the GeoJSON format that puts each Feature object on its own line, so that GeoJSONL files can be read more efficiently by certain programs. Placemark always reads and writes files from start to end, so the performance differences attributed to GeoJSONL do not apply. But if you are interacting with a service or program that produces or expects GeoJSONL, you can import and export this file format.
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