Measures the extent to which an index is fragmented by being built in multiple indexing jobs.

Compressing an index eliminates fragmentation. Fragmentation of an index increases the size of the index and slows searching, but the effect is generally not noticeable unless the fragmentation is severe. Indexing documents in very small batches can produce a severely fragmented index, but otherwise index fragmentation can be disregarded.

Compressing 

IndexInfo::Fragmentation Data Member