Contents classified as cross-client

What causes this issue?

The contents of the selected Switch BC Set have been classified
according to the first data record, as "cross-client".
You can only put data records with this classification in the Switch BC
Set (e.g. no client-specific data records).

System Response

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

How to fix this error?

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.