What is the function of segmentMerge.oldNewDocNums?

[KaniuBillows]

I‘m trying to read about the code.
But I am not very clear about the function of this data structure after the merger. Can someone help me explain its function? Thank you very much!

https://github.com/blevesearch/bleve/blob/5939277722260910a9d6831259487c3ae715ba5f/index/scorch/merge.go#L459C1-L498C3