Field & does not lie within customer namespace

What causes this issue?

Fields of <DS:TX.KUNDEN-INCLUDE>customizing includes</> and
<DS:TX.DDDOK_APPEND_STRUKTUR>append structures</> should lie in the
customer namerange. You can then be sure that there will be no
conflicts with SAP field names in future Release upgrades.
The customer namerange for fields is YY and ZZ.

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?

If they are new fields or fields which are only used in structures, you
are recommended to change the field names and the reports which access
them.
The field name for fields of database tables cannot be changed without
the corresponding data being lost. It is only necessary to change the
field name if the same name is used by SAP.

Error message extract from SAP system. Copyright SAP SE.