Services are meant to be building blocks for business processes and could provide functionality that is potentially relevant to multiple applications. In order to fully realize these benefits, it is critical that variations in service behavior are supported.
What are some of the aspects that vary in a service capability? From a functional perspective, a service could support different behavior depending on the characteristics of the invoking client, the nature of data that is worked on, or both. Additionally, a service capability could also provide optional filters and preferences that allow for varying behavior (e.g. a service could break down data into multiple chunks, and returning only those that the client is interested in). Understand your domain to identify service capabilities that could support client preferences.
From a non-functional perspective, response time, number of concurrent requests, volume of requests within a time window, and total number of requests across clients etc. are aspects that could be managed. In addition to response time, data format (supporting plain text, JSON, in addition to XML) could be varied as well to ease integration.