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 numerous vectors, such as interviews, questionnaires, videos or 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 article, we present the usage tracking language (ULT). This language was designed to be generic and we present an instantiation of a part of it with IMS-Learning Design, the representation model we chose for our three years of experiments. At the end of the article, we describe several use cases of this language, based on our experimentations. (Contains 4 tables and 10 figures.)