# Subscriber keys, contact keys and contact IDs

*Marketing Cloud Engagement Foundations Study Guide · Marketing Cloud Engagement Basics · Last updated November 3, 2026 · https://www.certifyforce.com/study-guide/marketing-cloud-engagement-foundations/marketing-cloud-engagement-basics/subscriber-keys-contact-keys-and-contact-ids*

Why subscriber key and contact key are one value you choose, why the Contact ID is a back-end number, how to pick a stable key, what the key lets you do, and why contacts outnumber subscribers.

- Certification: Salesforce Marketing Cloud Engagement Foundations
- Exam section: Marketing Cloud Engagement Basics
- Study guide: https://www.certifyforce.com/study-guide/marketing-cloud-engagement-foundations

**Official exam objective:**

> Differentiate between subscriber keys, contact keys, and contact IDs to uniquely identify subscribers.

## What the exam expects

- Explain that subscriber key and contact key are the same value, named differently by Email Studio and Contact Builder.
- Distinguish the key you assign from the Contact ID that Salesforce assigns on the back end.
- Choose a unique, stable, non-channel value as the key, such as the 18-digit Salesforce record ID.
- List what a subscriber key makes possible, and predict what inconsistent keys do to contact counts.
- Explain why every subscriber is a contact but not every contact is a subscriber.
- Key terms: **subscriber key** (the value you choose that uniquely identifies a subscriber); **contact key** (the same value in Contact Builder, constant across channels); **Contact ID** (a number Salesforce uses to identify a contact on the back end); **contact** (anyone you message on any channel); **subscriber** (a person who opted in to or belongs to a channel).

## Key facts

| Fact | Detail | Source |
|---|---|---|
| Two names, one value | Subscriber key in Email Studio, contact key in Contact Builder | [Contact Model Relationships](https://trailhead.salesforce.com/content/learn/modules/marketing-cloud-contact-management/understand-contacts-and-contact-model-relationships) |
| Contact ID | Back-end number Salesforce uses | [Contact Model Relationships](https://trailhead.salesforce.com/content/learn/modules/marketing-cloud-contact-management/understand-contacts-and-contact-model-relationships) |
| Key length | Up to 254 English characters | [Subscriber Key in Engagement](https://help.salesforce.com/s/articleView?id=mktg.mc_es_subscriber_key.htm&type=5&language=en_US) |
| Email as key | Avoid it | [Subscriber Key in Engagement](https://help.salesforce.com/s/articleView?id=mktg.mc_es_subscriber_key.htm&type=5&language=en_US) |
| Salesforce records | 18-digit ID stored as the key during Connect sends | [Subscriber Key](https://help.salesforce.com/s/articleView?id=mktg.mc_co_subscriber_key.htm&type=5&language=en_US) |
| All Subscribers entry | Only after something is sent | [Contact Model Relationships](https://trailhead.salesforce.com/content/learn/modules/marketing-cloud-contact-management/understand-contacts-and-contact-model-relationships) |
| Sendable data extensions | Must contain the subscriber key | [Contact Model Relationships](https://trailhead.salesforce.com/content/learn/modules/marketing-cloud-contact-management/understand-contacts-and-contact-model-relationships) |

## Check yourself

- What is the maximum length of a subscriber key?
- When you import or create a subscriber, what does Engagement use the subscriber key to decide?
- When you export subscribers, which value appears in the output whatever criteria you searched on?
- Why would a loyalty ID such as "GX(7781)" fail as a subscriber key?

## Sources

- [Contact Model Relationships and Data Model (Trailhead)](https://trailhead.salesforce.com/content/learn/modules/marketing-cloud-contact-management/understand-contacts-and-contact-model-relationships)
- [Subscriber Key in Marketing Cloud Engagement](https://help.salesforce.com/s/articleView?id=mktg.mc_es_subscriber_key.htm&type=5&language=en_US)
- [Subscriber Key](https://help.salesforce.com/s/articleView?id=mktg.mc_co_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)
- [Contacts and Devices](https://help.salesforce.com/s/articleView?id=mktg.mc_mp_contact_or_subscriber_key.htm&type=5&language=en_US)

## About the full unit

The "Core concepts", "Exam traps" and "Worked scenario" sections of this unit, its 7 linked practice questions, and unlimited timed mock exams are included with the Marketing Cloud Engagement Foundations pack (USD $19): https://www.certifyforce.com/practice-exams