You can find this information in the Nastran Linear Static Analysis User Guide:
RBE2- A rigid body connected to an arbitrary number of grid points. The independent degrees of freedom are the six components of motion at a single grid point. The dependent degrees of freedom at the other grid points all have the same user-selected component numbers.
RBE1- A rigid body connected to an arbitrary number of grid points. The independent and dependent degrees of freedom can be arbitrarily selected by the user.
