# Contacts across email, SMS and MobilePush channels

*Marketing Cloud Engagement Developer Study Guide · Data Modeling · Last updated November 3, 2026 · https://www.certifyforce.com/study-guide/marketing-cloud-engagement-developer/data-modeling/contacts-across-email-sms-and-mobilepush-channels*

How one contact key ties email addresses, mobile numbers and MobilePush devices to a single contact, how devices and contact keys behave in MobilePush, how MobileConnect contacts are loaded, and why child business units see different contacts.

- Certification: Salesforce Marketing Cloud Engagement Developer
- Exam section: Data Modeling
- Study guide: https://www.certifyforce.com/study-guide/marketing-cloud-engagement-developer

**Official exam objective:**

> Describe how contact records relate across channels.

## What the exam expects

- Explain how a single contact record holds the email address, mobile number and push device of one person, and what splits a person into several contacts.
- Distinguish a subscriber from a contact.
- Predict how MobilePush assigns and changes contact keys, and how devices move between contacts.
- Prepare MobileConnect and MobilePush contact loads that join the same contact as email.
- Explain why a contact visible in one Enterprise 2.0 child business unit is missing from another.
- Key terms: **contact key** (also called subscriber key; the unique identifier that associates a contact's cross-channel addresses and subscriptions); **device ID** (a unique identifier for one mobile device); **subscriber** (a person who opted in to specific channels); **single view of the customer** (one contact record holding every channel address and its tracking).

## Key facts

| Fact | Detail | Source |
|---|---|---|
| Contact key across channels | The same key can be used for email, SMS and push | [Contacts and Devices](https://help.salesforce.com/s/articleView?id=mktg.mc_mp_contact_or_subscriber_key.htm&type=5&language=en_US) |
| No key at SDK initialization | MobilePush generates a UUID | [Contacts and Devices](https://help.salesforce.com/s/articleView?id=mktg.mc_mp_contact_or_subscriber_key.htm&type=5&language=en_US) |
| Changing a contact key | Allowed at any time | [Contacts and Devices](https://help.salesforce.com/s/articleView?id=mktg.mc_mp_contact_or_subscriber_key.htm&type=5&language=en_US) |
| Devices per contact | Many devices per contact, one contact per device | [Contacts and Devices](https://help.salesforce.com/s/articleView?id=mktg.mc_mp_contact_or_subscriber_key.htm&type=5&language=en_US) |
| New login on a device | Device moves to the new key in about 15 minutes | [Contacts and Devices](https://help.salesforce.com/s/articleView?id=mktg.mc_mp_contact_or_subscriber_key.htm&type=5&language=en_US) |
| MobileConnect import | .csv up to 20 MB with _MobileNumber, _Locale, _ContactKey | [Import File Guidelines](https://help.salesforce.com/s/articleView?id=mktg.mc_moc_prepare_file_for_import.htm&type=5&language=en_US) |
| SMS subscriptions | Specific to each short or long code | [Import Contacts](https://help.salesforce.com/s/articleView?id=mktg.mc_moc_import_contacts_to_a_standard_list_or_all_contacts.htm&type=5&language=en_US) |
| Business unit visibility | Some contacts are available only to one business unit | [Contact Count Determination](https://help.salesforce.com/s/articleView?id=mktg.mc_cab_contact_definition_and_count_determination.htm&type=5&language=en_US) |

## Check yourself

- Why can a push notification reach a phone that didn't meet the filtered list's criteria?
- When does an SMS opt-out on one short code stop messages sent from another code?
- Why shouldn't a MobilePush Device ID be treated as the identity of a physical phone?
- What must be true to import MobilePush contacts from a data extension?

## Sources

- [Contacts and Devices](https://help.salesforce.com/s/articleView?id=mktg.mc_mp_contact_or_subscriber_key.htm&type=5&language=en_US)
- [Set the Subscriber Key or Contact Key](https://help.salesforce.com/s/articleView?id=mktg.mc_mp_use_case_subscriber_key.htm&type=5&language=en_US)
- [Contact Definition and Count Determination in Contact Builder](https://help.salesforce.com/s/articleView?id=mktg.mc_cab_contact_definition_and_count_determination.htm&type=5&language=en_US)
- [Contact Builder in Marketing Cloud Engagement](https://help.salesforce.com/s/articleView?id=mktg.mc_cab_contact_builder.htm&type=5&language=en_US)
- [Contact Builder Best Practices](https://help.salesforce.com/s/articleView?id=mktg.mc_cab_contact_builder_best_practices.htm&type=5&language=en_US)
- [Audiences (MobilePush)](https://help.salesforce.com/s/articleView?id=mktg.mc_mp_contacts.htm&type=5&language=en_US)
- [Who Receives Your Push Notification](https://help.salesforce.com/s/articleView?id=mktg.mc_mp_who_receives_your_message.htm&type=5&language=en_US)
- [Import Contacts to a Standard List or All Contacts](https://help.salesforce.com/s/articleView?id=mktg.mc_moc_import_contacts_to_a_standard_list_or_all_contacts.htm&type=5&language=en_US)
- [Import File Guidelines](https://help.salesforce.com/s/articleView?id=mktg.mc_moc_prepare_file_for_import.htm&type=5&language=en_US)
- [Use MobilePush with Automation Studio](https://help.salesforce.com/s/articleView?id=mktg.mc_mp_use_mobilepush_with_automation_studio.htm&type=5&language=en_US)

## About the full unit

The "Core concepts", "Exam traps" and "Worked scenario" sections of this unit, its 8 linked practice questions, and unlimited timed mock exams are included with the Marketing Cloud Engagement Developer pack (USD $19): https://www.certifyforce.com/practice-exams