A single client can use multiple retainers and conversely a single retainer can also serve multiple clients. Client-Servers today function on the ‘request-response’ model where a client needs a task done and sends a request to the retainer which is accomplished by the server.
A computer designated as ‘server-class hardware’ means that is a specialized device for running server functions. Though the implication leads to visions of large, powerful and reliable computer devices, in reality, a retainer may be a cluster of relatively simple components.