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Coverage criteria provide a useful and widely used means to guide software testing; however, indiscriminately pursuing full coverage may not always be convenient or meaningful, as not all entities are of interest in any usage context. We aim at introducing a more meaningful notion of coverage that takes into account how the software is going to be used. Entities that are not going to be exercised by the user should not contribute to the coverage ratio. We revisit the definition of coverage measures, introducing a notion of relative coverage. According to this notion, we provide a definition and a theoretical framework of relative coverage, within which we discuss implications on testing theory and practice. Through the evaluation of three different instances of relative coverage, we could observe that relative coverage measures provide a more effective strategy than traditional ones: we could reach higher coverage measures, and test cases selected by relative coverage could achieve higher reliability. We hint at several other useful implications of relative coverage notion on different aspects of software testing.