Master record & & is classed

What causes this issue?

The master record you processed is classified.
The system creates a new version of this master record, as you have
either created a new validity period or changed an already existing
validity period with history.
The classification is identical for <ZH>all versions</> of a master
record, because it refers to the object directly.
For this reason, you should - if you work in the classification system
with <DS:GLOS.reference characteristic>reference characteristics</> -
<ZH>not</> change those fields of the master record that
exist in the class as reference characteristics
already contain a value

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?

Error message extract from SAP system. Copyright SAP SE.