You are trying to fetch the Background Elements in the SuccessFactors SFAPI CompoundEmployee to integrate with your 3rd party system or SAP ERP.
- SAP SuccessFactors Employee Central Payroll
- SAP SuccessFactors Employee Central
- CompoundEmployee SFAPI
- Integration with SAP ERP S4 On premise or On Cloud
Standard replication content which uses CompoundEmployee API to pull data from SF, does not support background elements as this data is not relevant for payroll.
In another words, the OData.Background_xxxx or SFAPI.Background_xxxx entities fields cannot be added in the SFAPI Compound Employee response payload.
You can see the fields selectable for the CompoundEmployee in one table inside the SFAPI CE handbook or also under Admin center > SFAPI Data Dictionary.
Other possibility would be adding custom MDF entities inside your SFAPI CE response, for that, use the instructions of the handbook or this KBA 2597777 - How to add custom MDF entities in the SFAPI CompoundEmployee
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