In the context of SAP S/4HANA, SAP HANA Cloud Platform serves as an extension platform and agility layer.
It is possible to build specific capabilities extending the scope of SAP S/4HANA by either integrating non-SAP functions or building your own capabilities. The cloud platform not only serves as the development platform but also as the runtime foundation for the developed solutions.
The extensions built on the cloud platform can run against both cloud and on-premise deployments of SAP S/4HANA.