A SimDM/Project is an aggregation of SimDM/Resource-s that belong together, for example because they have been produced together in the course of a scientific project. 
Examples are parameter studies where a large number of simulations is run with slightly varying parameter settings. But also a single large simulation with a number of post-processing results can be gathers in this way. 

It is assumed that SimDM/Project are generally "big enough" to qualify to be represented as a full-fledged Registry/Resource in an IVOA Registry. This possibility was on of the reasons to add this concept to the model.