What does vertical scaling an application deployment describe best?

Adding or removing *applications* relates to deploying or de-provisioning different services, not scaling an existing application.

Adding or removing *node instances* to the cluster is scaling the underlying infrastructure, which can support horizontal application scaling, but is not vertical application scaling itself.

Vertical scaling, also known as 'scaling up' or 'scaling down,' involves increasing or decreasing the CPU, memory, or disk resources allocated to an existing instance of an application to handle varying workloads and improve its performance, without changing the number of instances.

Adding or removing *application instances* of the same application is horizontal scaling, not vertical scaling, as it involves changing the number of running copies.