could you please explain the difference between the "Contact KEY" and the "Contact ID" and how do they relate each other (if so)?
If I had in the contact builder a data extension linked to the contact via "Contact KEY" (e.g. e-commerce orders) and an other data extension linked to the contact via "Contact ID", (e.g. mobile behavior) how does the system understand how to relate the data to the same contact?
The explanation given here https://help.salesforce.com/apex/HTViewSolution?id=000232653&language=en_US is not very useful.