Conclusion is,
Underscore is not supported in the Financials ERP and it's used as a segment delimiter in the Financials UI therefore end-user can not create a resource with reserved "_" character.
As of today, API allows creating resources with "Underscore" in their reference values. This has been registered as a bug in API with POST operations and will be fixed. API should not allow using "_" for resource identifiers. This may lead wrongfully referenced inventories in the UI/ DB transactions therefore you should avoid to use underscore while creating resources via API until this bug is fixed. (Once this bug is fixed, API won't allow underscore usage complying with the Financials ERP Data Structure.)
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