The use of modeling, simulation and visualization techniques in scientific and technical domains has led to the co-existence of a large diversity of external representations that, when deployed in collaborative work settings, can be designated by the term "shared representations". This contribution will focus on dyadic cognitive and triadic semiotic perspectives on the issue of interpretation and construction of shared representations. We propose a typology of external representations as a basis for further study of shared representations. As illustrated by examples from an existing corpus, the framework allows describing co-occurrence and superimpositions of different types, as well as transitions from one type to another. In the final section, we anticipate the exploitation of the framework in the light of the design of collaborative systems.