This paper intends to clarify recent efforts proposed by the Berkeley school in order to give a formal semantics to the Ptolemy toolbox. We show that the proposed semantics is indeed a generalisation of a Kahn semantics based on tag systems. We make this proposal a precise one by showing for several Ptolemy domains what is the corresponding tag system and how operators can be defined. We first remark that this semantics doesn’t need ”absent” values. We also discuss some domains which do not obey this semantics. We then discuss some advantages of this semantics, namely that it provides heterogeneity and distribution for free. In particular, we show that while the semantics is naturally expressed in terms of actors, directors are not essential for semantical purposes and are only justified by simulation efficiency purposes. We conclude that in most cases, directors do not play an important part in the definition of a Model of Computation and Communication.