We are trying to mass update customer addresses

We are in the process of updating addresses for over 2,200 customers using the Customer Import Template. A query was created to fetch all the necessary fields required for the update. While we successfully retrieved and updated relevant information for approximately 1,100 customers, the remaining 1,100 records are encountering an issue. Specifically, the field "PARTY_SITE_ORIGINAL_SYSTEM_REFERENCE" is blank for these customers.

Our primary question is: Why is this field missing data for these specific 1,100 customers?

how can we fetch this Data/ is it even present in the system what should be our next course of action if we are not able to find them.