Placemark supports an id for every feature. This is in accordance with the GeoJSON "id" member, which can either be a string or a number value. It's recommended that ids are unique, but this isn't enforced, because GeoJSON documents with duplicate ids are valid.
Every feature also has a system ID, which is guaranteed to be unique across all of Placemark: this is a UUID. System IDs can't be updated or controlled - they're created when features are added. You can use system IDs in order to refer to features at the API level - every feature has its own endpoint in the REST API.
You can optionally include System IDs in GeoJSON exports by selecting the option in the export dialog.
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