What's the difference between "geo", "geo1", and "geo2"?

[jievince]

I'm reading the source code of geo spatial.

class RocksDBGeoIndex final : public RocksDBIndex, public geo_index::Index {
  friend class RocksDBSphericalIndexIterator;

 public:
  RocksDBGeoIndex() = delete;

  RocksDBGeoIndex(IndexId iid, arangodb::LogicalCollection& collection,
                  arangodb::velocypack::Slice const& info, std::string const& typeName);

  ~RocksDBGeoIndex() = default;

  IndexType type() const override {
    if ("geo1" == _typeName) {
      return TRI_IDX_TYPE_GEO1_INDEX;
    } else if ("geo2" == _typeName) {
      return TRI_IDX_TYPE_GEO2_INDEX;
    }
    return TRI_IDX_TYPE_GEO_INDEX;
  }

  bool pointsOnly() const { return (_typeName != "geo"); }