The default type of index created for a Neighbor Partition vector index is Inverted File Flat.

• Understand Inverted File Flat Vector Indexes
  The Inverted File Flat vector index is a technique designed to enhance search efficiency by narrowing the search area through the use of neighbor partitions or clusters.
• Inverted File Flat Vector Indexes Partitioning Schemes
  Inverted File Flat vector indexes support both global and local indexes on partitioned tables. By default, IVF indexes are globally partitioned by centroid.