thanks for your input Bob DuCharme. to be honest, it took me a veeery long time to take the time to look into what rdf are because this "semantic web" term souned like a fraud to me : I thought, "semantic" is what is related to meaning and meaning is a human thing, it did not make sense to use it about machine. Then I realised a lot of human competence are metaphorically used for machine, so why not this one. The question is what is exactly that it refers to, hence my question. In particluar, it is often not clear to me what is more "semantic" about the layer people call "semantic layer" , compared to other part of the system/architecture.