Different situations - like school and workplace - demand different forms of knowledge. Even more important, in particular for lifelong learning, are forms of knowledge we need for managing movements between those situations. To develop a better understanding of how to 'navigate' knowledge boundaries, this paper analyzes, firstly, interviews with scientists interpreting familiar and unfamiliar graphs. Our goal is to identify those forms of knowledge that should receive special attention in education. Secondly, the article elaborates - based on Peirce's semiotic - an epistemologically reflected semiotic model to describe the role and conditions of knowledge.