Data models or knowledge models? What do we often use in engineering? What does a difference in the technical field? A short presentation: knowledge models offers pre-defined solutions, delivering knowledge about the process we are interested in. Data models put in an accepted format a piece of reality. The purpose of this paper is to underline the difference between the two notions and to sharpen the notion of knowledge components: explicit, implicit and tacit knowledge and the approach consists in a review of the science literature. It will be also presented the notion of surface-knowledge. The examples and all the work will be placed in the context of the manufacturing engineering and technology field.