THE SHORT ANSWER
Use an overlay to add reviewed values while retaining the original source. Join on stable product identifiers, define which fields the overlay may change, and recheck approvals when source evidence changes. A supplemental data source should have a clear purpose and a predictable precedence rule.
Decide which system owns each field
An overlay becomes risky when it can change everything. Start with a field policy: commercial facts such as price and stock usually need a current operational source, while a reviewed descriptive attribute may be suitable for a separate enrichment layer.
Record both the source value and the proposed value. Include the reason for the change and its evidence. This makes the improvement inspectable and lets a reviewer distinguish correction, formatting and genuinely new information instead of seeing one unexplained final cell.
Join records with stable identifiers
Use a stable product or variant key from the source you intend to augment. Titles, row positions and image filenames are fragile joins because ordinary merchandising edits can change them. Preserve identifier text, including leading zeros, through imports and spreadsheet exports.
Test unmatched and duplicate keys before applying any changes. An unmatched overlay is not harmless if the team assumes it was applied. A duplicate key can be worse, because one product may receive an ambiguous or unintended value.
Preview precedence and protected fields
Define whether a supplied overlay value replaces a source value, fills only an empty field or is held for review. Treat blank values deliberately: empty should not accidentally mean delete in one system and ignore in another. Preview the final value that the destination will receive.
Keep destination-specific behavior separate. A Google Merchant Center supplemental data source has its own processing rules; a custom transformation pipeline may behave differently. Reusing the same file name does not make the two workflows equivalent. Verify the receiving system’s current behavior.
Revisit approval when evidence changes
An approved value can become stale when the source product changes. Link the approval to relevant source fields or a source revision. A new material specification should reopen review of material-dependent text rather than preserving yesterday’s approval indefinitely.
Use a small exception queue for these changes. The goal is not to make every import a full manual review, but to focus attention where the original reason for approval no longer holds. Retain enough history to explain the released value later.
ILLUSTRATIVE WORKED EXAMPLE
Make it concrete.
Illustrative example: a merchant approves a fabric composition overlay for SKU 00128 using its supplier sheet. A later import changes the supplier reference. The workflow keeps the original values visible and asks for review before reusing the composition in the next channel release.
Your practical checklist
- Choose stable text identifiers for overlay joins.
- Define allowed fields and protected commercial facts.
- Review unmatched and duplicate keys.
- Specify blank-value and precedence behavior.
- Reopen approvals when relevant source evidence changes.
Common questions
Can a supplemental feed replace the primary feed?
Its role depends on the receiving system. Plan it as an augmentation with an explicit join and processing policy rather than assuming it behaves as a standalone catalog.
Should every overlay overwrite a non-empty value?
No. Decide per field whether the overlay fills gaps, corrects a value or requires explicit review.
Sources & scope
Official references checked 29 October 2026. Examples are illustrative, not customer results. Platform requirements can change; use current destination documentation for implementation decisions.