Extension & is not assigned to any basic type

What causes this issue?

No basic type yet assigned to extension &v1&

System Response

The processing of extension &v1& in the IDoc type editor is limited in
this status.
Extension &v1& can, however, be used in IDoc processing (inbound or
outbound).

How to fix this error?

Please assign a basic type to the extension in the IDoc type editor.
After you have done this, you can perform further processing on the
extension in the IDoc type editor.

Error message extract from SAP system. Copyright SAP SE.