Private and protected components will automatically be made public

What causes this issue?

The interface contains components that are declared as protected or
private. However, interface components must always be public in ABAP
Objects.
When you confirm this message, any private or protected interface
components will be converted to public.
The system will create a transport entry with the form R3TR INTF
<interface>.

Error message extract from SAP system. Copyright SAP SE.