Table & also maintained by the customer (table class &)

What causes this issue?

You are about to change the contents of a table that is also maintained
by the customer.

System Response

The system issues a warning, as there is a danger that your change may
cause customer settings to be overwritten when the next update is
imported (in category M). If the table belongs to category G, the entry
is only imported into the customer system if it is a new entry (no
overwriting with category G).

How to fix this error?

Consider how your change will affect the customer.

Error message extract from SAP system. Copyright SAP SE.