This paper addresses the problem of expression and sharing of domain-specific constraints and knowledge in engineering design. Pierce's theory of signs allows understanding why purely graphical 3D CAD systems partially fail to support design teams during design review meetings and more generally during collaborative design episodes. This theory can help us to go further in analysing annotation systems as symbolic systems of signs and we draw a parallel with the 2D standard drawing system, as they have the same semiotic nature. We show that semantic annotations can be considered to improve the semiotic richness in 3D CAD representations. A case study illustrates the need for co-operation support to multi disciplinary design teams. In this paper we support the idea that future design tools should integrate both symbolic and iconic types of representations so that the designers could freely build shared representations taking advantage of the whole symbolic-iconic spectrum. We propose future developments reconciling the graphical precision of 3D digital models with the semantic richness of symbolic systems of signs.