In the standard testing theory of DeNicola-Hennessy one process is considered to be a refinement of another if every test guaranteed by the former is also guaranteed by the latter. In the domain of web services this has been recast, with processes viewed as servers and tests as clients. In this way the standard refinement preorder between servers is determined by their ability to satisfy clients.

But in this setting there is also a natural refinement preorder between clients, determined by their ability to be satisfied by servers. In more general settings where there is no distinction between clients and servers, but all processes are peers, there is a further refinement preorder based on the mutual satisfaction of peers.

In this talk we are concerned with the behavioural characterisations of the server and the client preorders. Acceptance sets are enough to capture the server preorder, but not the client one. To characterise the client preorder we introduce the notion of "usability", and combine it with the one of acceptance sets. This is enough to characterise also the peer preorder.