The file format version, a negative number.

This format adds a checksum at the end of the file to ensure all bytes were successfully written.

This format adds the deletion count for each segment. This way IndexWriter can efficiently report numDocs().

This format adds optional per-segment String dianostics storage, and switches userData to Map

This format adds the boolean hasProx to record if any fields in the segment store prox information (ie, have omitTermFreqAndPositions==false)

This format adds details used for lockless commits. It differs slightly from the previous format in that file names are never re-used (write once). Instead, each file is written to the next generation. For example, segments_1, segments_2, etc. This allows us to not use a commit lock. See file formats for details.

This format allows multiple segments to share a single vectors and stored fields file.

This format adds a "hasSingleNormFile" flag into each segment info. See LUCENE-756 for details.

This format adds optional commit userData (String) storage.