opaque user identifier

https://medium.com/swift-blondie/opaque-return-type-in-swift-f5a8887c6be6

What is an opaque ID? The opaque ID is unique to the relationship between the customer's Apple ID and the business’s business ID. A customer has a different opaque ID for every chat they have using Messages for Business. The customer decides if and when to share personal identifying information with the business. For more information, see Apple Platfrom Security

https://register.apple.com/resources/messages/messaging-documentation/faq

You can get more information from the PDF on page 159：Secure Business Chat using the Messages app