A. Use Customer 360 Data Manager to assign the Global Party ID and use it as a primary key across all systems including the new Subscriber ID in Marketing Cloud.

B. Send the Marketing Cloud Subscriber Key to Service Cloud and B2C Commerce to be held for reference.

C. Migrate the Subscriber Key in Marketing Cloud to be the Service Cloud Contact or Person Account ID.

D. Using Service Cloud as a central point hold unique identifiers from all systems including the Service Cloud Contact or Person Account ID and B2C Commerce CustomerNo and Customer ID