what does option "persistInHeap" mean in hadoop-index?

hi, guys
Excuse me, what does the option “persisitInHeap” mean?

Does it related to memory consumption / disk consumption in reducer processes?

I didn’t found any document about this option.

It is a secret flag to control the data structure to store events in the write optimized format. We were experimenting with using heap and off-heap memory for writes when ingesting real-time data.