# Contact key choice, billable contacts and Contact Delete

*Marketing Cloud Engagement Consultant Study Guide · Data Modeling and Management · Last updated November 3, 2026 · https://www.certifyforce.com/study-guide/marketing-cloud-engagement-consultant/data-modeling-and-management/contact-key-choice-billable-contacts-and-contact-delete*

What a contact key choice costs or breaks: stable, channel-free keys, subscriber key behaviour and migration, how billable contacts are counted, what creates contacts without channels, and how to clean them up safely with Contact Delete.

- Certification: Salesforce Marketing Cloud Engagement Consultant
- Exam section: Data Modeling and Management
- Study guide: https://www.certifyforce.com/study-guide/marketing-cloud-engagement-consultant

**Official exam objective:**

> Evaluate implications on data model when choosing contact identity.

## What the exam expects

- Recommend a contact key that is unique, channel-free, stable and safe to expose, and reject keys that break those rules.
- Explain how the subscriber key drives All Subscribers, repeated addresses, preference updates and tracking exports, and what a migration to subscriber keys requires.
- Calculate billable contacts and explain where email and mobile identities are managed in Enterprise 2.0.
- Diagnose what created contacts without channel addresses from the key format and the feature in use.
- Plan a safe Contact Delete cleanup.
- Explain the Contact Delete phases, where deletion is enabled and what it leaves for the customer.
- Key terms: **contact key** (single value identifying a contact account-wide, identical to the Email Studio subscriber key); **billable contact count** (de-duplicated union of All Contacts, populations and hidden triggered send lists); **channel-less contact** (in All Contacts with no email, mobile, push or LINE address); **hidden managed list** (where triggered sends keep recipients not added to All Subscribers); **Contact Delete** (process that deletes contacts across all subscriber types).

## Key facts

| Fact | Detail | Source |
|---|---|---|
| Subscriber key length | 254 English characters | [Subscriber Key](https://help.salesforce.com/s/articleView?id=mktg.mc_es_subscriber_key.htm&type=5&language=en_US) |
| Data obscurity | Subscriber key is the only field not encrypted | [Subscriber Key](https://help.salesforce.com/s/articleView?id=mktg.mc_es_subscriber_key.htm&type=5&language=en_US) |
| Disallowed characters | Hyphen on one page, @ on the other | [Contact Builder Best Practices](https://help.salesforce.com/s/articleView?id=mktg.mc_cab_contact_builder_best_practices.htm&type=5&language=en_US), [Subscriber Key](https://help.salesforce.com/s/articleView?id=mktg.mc_es_subscriber_key.htm&type=5&language=en_US) |
| Merging contacts | Not possible in Engagement | [Contact Builder Best Practices](https://help.salesforce.com/s/articleView?id=mktg.mc_cab_contact_builder_best_practices.htm&type=5&language=en_US) |
| Subscriber key migration | Up to one week with no sends or imports | [Identify Unique Subscribers Using Keys](https://help.salesforce.com/s/articleView?id=mktg.mc_es_migrate_your_account_to_use_the_subscriber_key_feature.htm&type=5&language=en_US) |
| Billable count | All Contacts + populations + hidden lists, de-duplicated by key | [Contact Counts](https://help.salesforce.com/s/articleView?id=000393417&language=en_US&type=1) |
| Email vs mobile scope | All Subscribers at EID level; mobile at business unit level | [Contact Counts](https://help.salesforce.com/s/articleView?id=000393417&language=en_US&type=1) |
| Synced records without a channel | Modified date 01/01/0001 12:00 AM | [Contact Definition and Count Determination](https://help.salesforce.com/s/articleView?id=mktg.mc_cab_contact_definition_and_count_determination.htm&type=5&language=en_US) |
| Contact Delete suppression period | 2 days by default; set in days under Manage Settings | [Contact Deletion](https://help.salesforce.com/s/articleView?id=mktg.mc_cab_contact_deletion.htm&type=5&language=en_US), [Enable Contact Delete](https://help.salesforce.com/s/articleView?id=mktg.mc_cab_enable_contact_delete.htm&type=5&language=en_US) |
| Non-sendable data extensions outside a population | Not reached by Contact Delete | [Contact Deletion](https://help.salesforce.com/s/articleView?id=mktg.mc_cab_contact_deletion.htm&type=5&language=en_US) |
| Contact Delete scope | Deletes all subscriber types | [Permanently remove Contacts or Subscribers](https://help.salesforce.com/s/articleView?id=000313920&language=en_US&type=1) |

## Check yourself

- Which modified date do synchronized records without a channel show?
- When does a child business unit show contact counts in Enterprise 2.0?
- Why can Journey History fail to show which journey created an older channel-less contact?
- How can a MobilePush app avoid creating anonymous contacts before login?

## Sources

- [Contact Builder Best Practices](https://help.salesforce.com/s/articleView?id=mktg.mc_cab_contact_builder_best_practices.htm&type=5&language=en_US)
- [Subscriber Key in Marketing Cloud Engagement](https://help.salesforce.com/s/articleView?id=mktg.mc_es_subscriber_key.htm&type=5&language=en_US)
- [Identify Unique Subscribers Using Keys](https://help.salesforce.com/s/articleView?id=mktg.mc_es_migrate_your_account_to_use_the_subscriber_key_feature.htm&type=5&language=en_US)
- [Understanding Contact Counts and Breakdown](https://help.salesforce.com/s/articleView?id=000393417&language=en_US&type=1)
- [Contact Definition and Count Determination](https://help.salesforce.com/s/articleView?id=mktg.mc_cab_contact_definition_and_count_determination.htm&type=5&language=en_US)
- [Permanently remove Contacts or Subscribers](https://help.salesforce.com/s/articleView?id=000313920&language=en_US&type=1)
- [Contact Deletion in Contact Builder](https://help.salesforce.com/s/articleView?id=mktg.mc_cab_contact_deletion.htm&type=5&language=en_US)
- [Enable Contact Delete in Contact Builder](https://help.salesforce.com/s/articleView?id=mktg.mc_cab_enable_contact_delete.htm&type=5&language=en_US)

## About the full unit

The "Core concepts", "Exam traps" and "Worked scenario" sections of this unit, its 21 linked practice questions, and unlimited timed mock exams are included with the Marketing Cloud Engagement Consultant pack (USD $19): https://www.certifyforce.com/practice-exams