In my opinion the Purebasic help is clear, how it is handled with "Extends".
Even the translation with DeepL into German is clear.The optional Extends parameter allows to extends another structure with new fields. All fields found in the extended structure will be available in the new structure and will be placed before the new fields. This is useful to do basic inheritance of structures.
The new structure is extended with the fields of the structure given with Expends. And these will be added at the beginning of the new structure. The given extended structure will not be changed.Der optionale Extends-Parameter erlaubt es, eine andere Struktur um neue Felder zu erweitern. Alle Felder, die in der erweiterten Struktur gefunden werden, sind in der neuen Struktur verfügbar und werden vor den neuen Feldern platziert. Dies ist nützlich, um grundlegende Vererbung von Strukturen durchzuführen.