What it does
The instance an application binds when the redis store is configured. The plugin's deployment guide provisions service redis-cache in the mta descriptor and tags the instance cds-caching; SAP's service metadata records the plans the service offers, from a free plan for non-productive development to a premium one.
Remaining work
Provision an instance on a plan the subaccount is entitled to and bind it. Which plans a given subaccount may use is a project question, not a documented property of either the service or the plugin.