[Koha] Importing records into Koha 3.14

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.