Order FAQ: Why does the Customer ID property sometimes have the customer's email address and not the Shopping Cart Customer ID?

Explains why Customer ID property has an email address sometimes

When Unific lacks a "Customer ID" for a specific customer in its database, the email address associated with that order is used as a placeholder in the customer ID field. This method ensures that each order can still be uniquely identified by an identifier, which, in this instance, is the email address, due to the absence of a conventional Customer ID.

This applies to Contacts (Customers) or Deals (Orders) synced into HubSpot.