Workgroup 3 of ArchaeDyn II programme focuses its study on the diffusion systems of ancient products. In order to be able to structure data in GIS in an appropriate way, we propose a general theoretical modelling integrating the different components of the diffusion systems, and identifying their interactions and the factors affecting the location of products and their transfers. Three dimensions are considered: the Time, the Space and the Function of places. A product's pathway can be apprehended efficiently by distinguishing spatial entities as well as functional entities. This modelling highlights the fact that the approach through the simple notion of "site" is not sufficient to study the products' diffusion without taking into account the nature and the role of the places. On the basis of this model, we propose a conceptual data model (created with the help of the HBDS method) that will finally lead to the creation of a "three-dimensional" geodatabase.