"Data Extension" is the generic term, but SFMC supports several DE types with different behaviors. Knowing the distinctions saves you from trying to send email from a lookup DE or forgetting to refresh a Filtered DE before a campaign.

Here's the reference we hand to every new engineer.

1. Sendable DE

The main type used for sending email. Requirements:

• Is Sendable = true in Properties
• A field of type EmailAddress
• A Send Relationship mapping a field (usually Primary Key) to the Subscriber Key

This is the DE you pick when you send a campaign or set up a Journey with Data Extension entry source. No send can happen without one.

2. Lookup DE (Non-sendable)

Reference data that AMPscript Lookup() pulls into email templates at render time. Common examples:

• Sales rep directory: SalesRep_DE with RepID, name, email, phone
• Product catalog: Product_DE with SKU, name, description, price
• Store locations: Store_DE with StoreID, address, hours

Not sendable, no EmailAddress field needed. Used by the email, not as the source of the send.

3. Filtered DE

A DE created by applying a Data Filter to another DE - point-and-click segmentation without SQL. Example: "subscribers from Master_DE where MemberTier = Gold" produces a Gold-only Filtered DE.

Important: Filtered DEs need to be refreshed to reflect the current state of the source DE. They don't auto-update.

Refresh via:

• Filter Activity in Automation Studio (scheduled or ad hoc)
• Manually in Email Studio

If the source DE changes (new imports, updates) but the Filtered DE isn't refreshed, the Filtered DE holds stale data. Campaigns targeting it send to outdated segments.

For anything more complex than single-attribute filtering (joins, calculations), use SQL Query Activity writing to a standard DE, not a Filtered DE.

4. Random DE

Splits a source DE into N equal random chunks. Use case: A/B/N testing where you need 10 equal groups to test 10 email variants.

No SQL needed. Configure the split percentages in the UI; SFMC assigns rows randomly.

Caveats:

• The "random" assignment happens at creation; refreshing re-randomizes, potentially putting the same subscriber into different buckets.
• For more sophisticated random assignment (e.g. Journey Builder's Random Split), the Journey tool is usually the better fit.

5. Shared DE

A DE placed in the Shared Data Extensions folder in the parent Business Unit. Multiple child BUs in the Enterprise account can access the DE without copying it.

Use when:

• The same reference data (product catalog, store list) is needed across multiple brands or regions.
• You want a single source of truth rather than syncing copies between BUs.

Access permissions are set via Shared Data Extension Permissions - which BUs can read/write.

Watch for unintended cross-BU writes: if two BUs can write to the same Shared DE, coordinate schemas and import schedules.

6. Send Log DE

A special DE that logs every email send - who received what, when, subject line, etc. Useful for:

• Audit requirements beyond SFMC's default 10-day retention
• Join with other data to build custom reporting
• Investigate specific send events

Created from the TriggeredSendDataExtension template when used with Triggered Sends.

Caveat: Test Sends don't write to Send Log. Only production sends do. If you're testing and expecting the Send Log to populate, it won't.

7. DE with Data Retention Policy

Any DE can have a retention policy configured on creation:

• Delete records older than X days - SFMC auto-purges rows past the threshold
• Delete data and DE after X days - whole DE goes away

Use for:

• Temporary event signup DEs (auto-purge 30 days after event)
• PII-heavy DEs that shouldn't retain data beyond a defined window
• Any Send Log or archive DE to prevent unbounded growth

Set retention at creation if possible. Adding it later works but doesn't apply retroactively to existing rows until the next automation evaluation.

Picking the right type on a new DE

NeedDE TypeSend email from this listSendableReference data AMPscript will look upLookupSimple attribute-based segmentFilteredA/B test random splitsRandomCross-BU shared referenceSharedAudit / custom trackingSend LogAuto-purge old dataDE with Retention Policy

Most projects end up with a mix: one or two Sendable DEs, several Lookup DEs, possibly a Shared DE for multi-brand setups, and retention policies on anything transient.

Takeaway

Naming the DE type right in your head before creating it prevents architecture rebuilds. The decision takes seconds; fixing an incorrectly-typed DE after it's been loaded with data can take a day.

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Designing SFMC data architecture? Our Salesforce team ships Data Extension layouts, shared-BU patterns, and retention strategies on production engagements. Get in touch ->

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