[Koha] Importing records into Koha 3.14

[Layale Bassil]

Hello,

I am performing  a batch import of bib records and items into Koha 3.14. I am storing items into field 952. I would like to understand the meaning of the option under "How to process items" = Replace items if matching bib was found (only for existing items).

Well nothing is being updated in the existing items. I would like to understand how the match on items is being done? What is the unique key that differentiate a 952 item from another.

Your help is very much appreciated.

Regards.