The purpose of annotations is to describe the content of “something” and they may be considered as meta-data. They are used for a while for text books, articles, hypertext documents and so on. We explore their usage in semantic-based and model-based interoperability, with the aim to make explicit the meaning and the structure of given models (artefacts) to enable not only their understanding, but also their exchange (and their possible transformation) between collaborating actors (human or machine). We propose categories and types of annotations helpful for expliciting the meaning of models and for easing their exchange within a collaborative context.