Message type: E = Error
Message class:  WRF_FOLUPART - Follow-Up/Replacement Material 
Message number: 026
Message text: Follow-up/replacement material & is a reference material
Follow-up/replacement materials <ZH>cannot</> belong to the following
material categories:
Generic material
Merchandise category material
Hierarchy material
Group material
Merchandise category reference material
Standard material
If the follow-up/replacement material belongs to one of the above
material categories, the system does not create any
follow-up/replacement material relationships.
INCLUDE ISR_CHECK
The system issues an error message and will not allow you to continue with this transaction until the error is resolved.
Error message extract from SAP system. Copyright SAP SE.
SAP Error Message: WRF_FOLUPART026 – Follow-up/replacement material & is a reference material
Cause: This error occurs when you try to assign a follow-up or replacement material that is defined as a reference material in SAP. Reference materials are typically used for informational or classification purposes and are not intended to be used as follow-up or replacement materials in processes like production orders, sales orders, or material master data maintenance. In other words, the system prevents you from linking a follow-up or replacement material if that material is flagged as a reference material because such materials are not valid for transactional or operational use. Explanation: Reference Material: A material flagged as reference is usually used for documentation, classification, or informational purposes. It is not a "real" material for production or sales. Follow-up/Replacement Material: These are materials that replace or follow up on another material in processes such as order processing, production, or inventory management. SAP enforces this check to
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WRF_FOLUPART025 Original article & is a reference material
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WRF_FOLUPART024 Follow-up/replacement material & is a value-only material
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WRF_FOLUPART028 No follow-up/replacement material specified
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