In the context of distance learning and teaching, the re-engineering process needs a feedback on the learners' usage of the learning system. The feedback is given by interviews, questionnaires, but in the majority of systems, it is given by log files. We consider that it is important to interpret tracks in order to compare the designer's intentions with the learners' activities during a session. In this paper, we present the usage tracking language - UTL. This language was designed to be generic and we present an instantiation with IMS-Learning Design, the representation model we chose for our three years of experiments. At the end of the paper, we develop an instance of a scenario for tracks analysis and we present the architecture of services around UTL.