Hi, To follow up on a discussion we had a few weeks ago, here is a brief description of a use case where multiple implementations of MPI could be used by a single process. This is more an illustration of the idea rather than a detailed description of a model. Let’s assume we are composing a multi-physics applications where each kernel, that has been developed separately, is an MPI application and requires a specific implementation of MPI (e.g., it requires a customized flavor of Trilinos, ultimately creating a dependency to OpenMPI). Let’s also assume that a composition library is based MPI sessions and rely on a specific implementation of MPI. It is then possible to compose the multi-physics application by coupling the two MPI codes with the composition library, the two MPI implementations being potentially different, in which case a single process is, in fact, using two different MPI implementations. Let me know if you have any question. Thanks,