# Contact keys, populations and contact counts

*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/contact-keys-populations-and-contact-counts*

How Contact Builder identifies one person with a single Contact Key, what adds to the contact count (populations, synchronized records, statuses, Enterprise 2.0 child business units) and how Data Designer relates data extensions with populations and cardinality.

- 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:**

> Configure account Contact model in Marketing Cloud.

## What the exam expects

- Choose and format a Contact Key that identifies one person across every channel, and predict what happens to records that arrive without one.
- Decide whether an individual, a status or a synchronized record adds to the account's contact count, including in Enterprise 2.0 accounts.
- Lower an inflated contact count with the documented lever for each source.
- Relate data extensions in Data Designer with populations, cardinality and compatible data types.
- Explain the one-way switch from an older subscriber data structure to the Contacts structure.
- Key terms: **Contact Key** (the one value identifying a contact, equal to the subscriber key); **contact count** (contacts from every source, deduplicated on ContactKey); **population** (a data extension linked 1:1 to the contact on ContactKey); **cardinality** (how two linked data extensions relate).

## Key facts

| Fact | Detail | Source |
|---|---|---|
| Contact Key | Identical to the Email Studio subscriber key; one per contact | [Contact Builder Best Practices](https://help.salesforce.com/s/articleView?id=mktg.mc_cab_contact_builder_best_practices.htm&type=5&language=en_US) |
| No ContactKey value | Defaults to the sending channel's address | [Contact Count](https://help.salesforce.com/s/articleView?id=mktg.mc_cab_contact_definition_and_count_determination.htm&type=5&language=en_US) |
| Converted synchronized lead | New ContactKey, separate contact, no merged attributes | [Contact Count](https://help.salesforce.com/s/articleView?id=mktg.mc_cab_contact_definition_and_count_determination.htm&type=5&language=en_US) |
| Journey Builder | Processing someone in a journey makes them a contact, even with no send | [Contact Count](https://help.salesforce.com/s/articleView?id=mktg.mc_cab_contact_definition_and_count_determination.htm&type=5&language=en_US) |
| Unsubscribed and deleted | Unsubscribed stay counted; deleted don't appear | [Contact Count](https://help.salesforce.com/s/articleView?id=mktg.mc_cab_contact_definition_and_count_determination.htm&type=5&language=en_US) |
| Synchronized records without a channel | ContactKey and 01/01/0001 12:00 AM only; disable sync to prevent | [Contact Count](https://help.salesforce.com/s/articleView?id=mktg.mc_cab_contact_definition_and_count_determination.htm&type=5&language=en_US) |
| Enterprise 2.0 | All Contacts count includes all child business units | [Contact Count](https://help.salesforce.com/s/articleView?id=mktg.mc_cab_contact_definition_and_count_determination.htm&type=5&language=en_US) |
| Populations | 1:1 on Contact Key or Contact ID; avoid more than three per account | [Create a Population](https://help.salesforce.com/s/articleView?id=mktg.mc_cab_create_a_population.htm&type=5&language=en_US), [Data Designer](https://help.salesforce.com/s/articleView?id=mktg.mc_cab_data_designer.htm&type=5&language=en_US) |
| Contacts structure switch | Can't return to the previous data structure | [Get Started with Contact Builder](https://help.salesforce.com/s/articleView?id=mktg.mc_cab_getting_started_with_contact_builder.htm&type=5&language=en_US) |

## Check yourself

- Why should a numeric customer number become a string before it serves as the contact key?
- Which data type can a Phone, Email Address or Date attribute link to in Data Designer?
- Why can changing a link's cardinality stop a scheduled send from reaching someone?
- Where does the Contacts Without Channel Addresses extract put its results?

## Sources

- [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 Best Practices](https://help.salesforce.com/s/articleView?id=mktg.mc_cab_contact_builder_best_practices.htm&type=5&language=en_US)
- [Data Designer in Contact Builder](https://help.salesforce.com/s/articleView?id=mktg.mc_cab_data_designer.htm&type=5&language=en_US)
- [Create a Population in Contact Builder](https://help.salesforce.com/s/articleView?id=mktg.mc_cab_create_a_population.htm&type=5&language=en_US)
- [Data Extensions in Contact Builder](https://help.salesforce.com/s/articleView?id=mktg.mc_cab_data_extensions.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)
- [Get Started with Contact Builder](https://help.salesforce.com/s/articleView?id=mktg.mc_cab_getting_started_with_contact_builder.htm&type=5&language=en_US)
- [Find Contacts Without Channel Addresses](https://help.salesforce.com/s/articleView?id=mktg.mc_cab_contacts_without_channel_address.htm&type=5&language=en_US)

## About the full unit

The "Core concepts", "Exam traps" and "Worked scenario" sections of this unit, its 9 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