While the regulation does not mandate specific proof that your non-relevant product is exempt, the best way to demonstrate its out-of-scope status to relevant authorities (like the Competent Authorities or customs) is through:
Product Classification / HS Code Documentation
Providing Customs-related documentation that definitively classifies your product under its correct Harmonized System (HS) or Combined Nomenclature (CN) code. Since synthetic rubber is not the commodity of concern, the relevant classification would demonstrate its non-relevance.
Material Composition Evidence
Having clear, verifiable documents, such as technical specifications, invoices, or suppliers' declarations, that confirm the product is made entirely of synthetic rubber and does not contain any of the relevant commodities (natural rubber, wood, cocoa, coffee, palm oil, soya, or cattle).