Which feature makes it possible for a storage administrator to add more than storage systems to a cluster?

'Scale horizontal' is not a recognized NetApp or industry-standard term; 'scale-out' is the correct and specific terminology used in ONTAP documentation.

Scale-out (horizontal scaling) in ONTAP allows administrators to add additional controller nodes to an existing cluster, expanding aggregate capacity, performance, and the number of supported SVMs without replacing existing hardware. ONTAP clusters support up to 24 nodes, making scale-out the architectural feature that enables growth beyond a single storage system.

Scale-up (vertical scaling) refers to adding more resources such as RAM, CPU, or disk shelves to an existing node rather than adding new nodes to the cluster.

'Scale vertical' is a synonym for scale-up and describes adding resources to an existing node, not adding new systems to the cluster.