A central part of configuring the Shopify app is choosing which customer identification method to use. This is done on the configuration page for the app, in the section shown here:
- Identify by email only (recommended): Search for customers using only email.
- Email first, then phone number: Search using first email and, if a match is not found, then by mobile phone number.
- Phone number first, then email: Search using first mobile phone number and, if a match is not found, then by email.
If you choose to search by email first and then phone number (the second option on the list) there is a risk of multiple matches that could result in a validation error. This is because email is always unique in Customer Engagement but phone number is not required to be, and it's possible that several contacts could have given the same number. Keep this in mind.