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.

1 Answer 1


A Contact Record has two identifiers:

  1. Contact Key: a user defined identifier
  2. Contact ID: a system defined identifier

When creating a Contact Record, you assign a Contact Key and Marketing Cloud will create a unique numeric Contact ID.

As a rule of thumb, you should always create a relationship to the Contact Record using Contact Key (this advice came from a lead Engineer on the Journey Builder product team).

Contact ID relationships are intended for system use and you probably would not know the Contact ID anyway (unless you created the Contact Record using the REST API or opened each Contact Record to see the Contact ID value).

Also note that in your Data Extension, the field that you are using to store your Contact Key will need to be a text field (not number). You can of course use a number in the field, but the field type has to be 'text' in order to create a cardinal relationship to the Contact Record.

Regarding your remark 'e.g. mobile behavior', I'm unclear what mobile behavior you are referring to. If you can expand on this, I'd be happy to elaborate further.

  • Hi @smdc01, does this answer your question? If so, please can you accept this answer for the benefit of others. If you need additional clarity, please let me know and I'd be happy to expand on my answer as required. Aug 1, 2016 at 22:50
  • Hi, I am sorry I was not able to reply earlier. Yep the answer makes sense, I would like to explain further on the mobile behavior. If I look at the Contact Builder, I do see that Mobile Connect Demographics is linked to the contact via Contact ID, and not Contact KEY ( see image here imgur.com/a/ZTwUA ). So the question is: how can I bind such info to my Contact if the notusing the Contact KEY as unique KEY? Thank you
    – sdmc01
    Aug 4, 2016 at 11:44
  • Yes, mobile subscribers are somewhat of an exception. The relationship that you are referring to is binding records from a MobileConnect list to the Contact Record. If you want to use the Send SMS Activity, you will need to follow this documentation and create an Import Definition to import the Data Extension records into a list. You should not need to touch Data Designer; the Contact Key relationship will be defined in your Import Definition. Aug 4, 2016 at 22:54

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