I am importing 2 CSV files and need to modify or skip existing contacts based on a parameter we assign at the point of sale.
1. First import will add the contact's contact info including the unique ID assigned at the time of sale 'Site ID'
2. Second import will update/modify that contact with additional information. Ideally, I want to match the 'Site ID' to ensure the correct records are being updated/modified

Right now the only options to skip/find existing are 'Contact ID' (assigned by Zoho) or email (which is historically unreliable as people forget which one they used).