# Cross-reference keys and lineage for traceability

*Platform Data Architect Study Guide · Master Data Management · Last updated October 26, 2026 · https://www.certifyforce.com/study-guide/platform-data-architect/master-data-management/cross-reference-keys-and-lineage-for-traceability*

How to keep a golden record traceable and its rules consistent: External ID fields and a child cross-reference object for source keys, lineage from each value to its source record, audit history of steward decisions, legacy audit fields set at load, and shared reference data through global value sets, stable picklist API names and classification metadata.

- Certification: Salesforce Platform Data Architect
- Exam section: Master Data Management
- Study guide: https://www.certifyforce.com/study-guide/platform-data-architect

**Official exam objective:**

> Given a customer scenario, recommend appropriate approaches and techniques to capture and maintain customer reference and metadata to preserve traceability and establish a common context for business rules.

## What the exam expects

- Store every source system key on the master: one External ID field per source, or a child cross-reference object when one master carries many keys.
- Keep lineage from each golden value to its source record and an audit history of merge decisions.
- Preserve legacy creation dates and users when loading, so audit and service rules still work.
- Give business rules a common context with shared reference data: global value sets, stable picklist API names and classification metadata.

Key terms: **External ID** (a field holding a record identifier from an outside system); **cross-reference object** (a child object with one row per source key of a master record); **lineage** (the trail from a value back to its source record); **global value set** (one restricted list of picklist values shared across fields); **audit fields** (Created By, Created Date and similar system fields).

## Key facts

| Fact | Detail | Source |
|---|---|---|
| External ID limit | Up to 25 per object (auto-number, email, number, text) | [Field Attributes](https://help.salesforce.com/s/articleView?id=platform.custom_field_attributes.htm&type=5) |
| External ID use | Update or upsert records through the API | [Field Attributes](https://help.salesforce.com/s/articleView?id=platform.custom_field_attributes.htm&type=5) |
| Merge effect | Duplicates deleted; related records reparented | [Merging Records](https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/langCon_apex_dml_examples_merge.htm) |
| Individual DMO | Carries Data Source ID and Data Source Object | [Unified Profiles](https://trailhead.salesforce.com/content/learn/modules/data-and-identity-in-salesforce-cdp/get-to-know-unified-profiles) |
| Unified Individual | Retains no source data; can't trace lineage alone | [Unified Profiles](https://trailhead.salesforce.com/content/learn/modules/data-and-identity-in-salesforce-cdp/get-to-know-unified-profiles) |
| Steward decisions | Full audit history for every decision | [MDM Capabilities](https://trailhead.salesforce.com/content/learn/modules/informatica-cloud-master-data-management-basics/discover-master-data-management-capabilities) |
| Audit fields at load | Set on create with Set Audit Fields upon Record Creation | [System Fields](https://developer.salesforce.com/docs/atlas.en-us.object_reference.meta/object_reference/system_fields.htm) |
| SystemModstamp | The only audit field you can't set | [System Fields](https://developer.salesforce.com/docs/atlas.en-us.object_reference.meta/object_reference/system_fields.htm) |
| Global value set | Restricted; no unapproved values even through the API | [Global Value Sets](https://help.salesforce.com/s/articleView?id=platform.fields_creating_global_picklists.htm&type=5) |
| Picklist dependencies | Built on value API names | [Dependent Picklists](https://help.salesforce.com/s/articleView?id=platform.fields_dependent_field_considerations.htm&type=5) |

## Check yourself

- Why does a merge break key-based upserts when keys live only in an External ID field on the master?
- When must legacy Created Date values be set?
- Why can't an audit report on Unified Individual alone show where a value came from?
- What stops users and integrations adding a new value to a global value set?

## Sources

- [Custom Field Attributes](https://help.salesforce.com/s/articleView?id=platform.custom_field_attributes.htm&type=5)
- [Merging Records](https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/langCon_apex_dml_examples_merge.htm)
- [Understand Unified Profiles and Their Impacts on Data Strategy](https://trailhead.salesforce.com/content/learn/modules/data-and-identity-in-salesforce-cdp/get-to-know-unified-profiles)
- [Discover Master Data Management Capabilities](https://trailhead.salesforce.com/content/learn/modules/informatica-cloud-master-data-management-basics/discover-master-data-management-capabilities)
- [Identify When to Use Master Data Management](https://trailhead.salesforce.com/content/learn/modules/informatica-cloud-master-data-management-basics/identify-when-to-use-master-data-management)
- [System Fields](https://developer.salesforce.com/docs/atlas.en-us.object_reference.meta/object_reference/system_fields.htm)
- [Create a Global Picklist Value Set](https://help.salesforce.com/s/articleView?id=platform.fields_creating_global_picklists.htm&type=5)
- [Dependent Picklist Considerations](https://help.salesforce.com/s/articleView?id=platform.fields_dependent_field_considerations.htm&type=5)
- [Data Classification Metadata Fields](https://help.salesforce.com/s/articleView?id=platform.data_classification_metadata_fields.htm&language=en_US&type=5)

## 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 Platform Data Architect pack (USD $19): https://www.certifyforce.com/practice-exams