Hi Barnabé,
The volume-create API is a Vserver API in Clustered ONTAP. You connect to a Vserver and then run the volume-create API to create a volume on that Vserver. Or, alternatively, you can connect to the Administrative Vserver on the Cluster Management interface and use Vserver tunneling (NaServer.setVserver()) to tunnel the volume-create API to your desired Vserver.
You might find the sample code included in the SDK helpful. It has an example of creating volumes using either method described above.
Thanks,
Ben