1. Method for designing an assembly of body parts or groups of body parts of an automotive vehicle body having a plurality of automotive modules using a digital simulation system, wherein each automotive module is an automotive vehicle body part or a group of automotive vehicle body parts, and wherein the plurality of automotive modules are functionally linked together by links, the method comprising: a step of integrating the automotive modules and the links, so as to form an assembly model for the assembly of body parts or groups of body parts, a step of modeling the automotive modules to create a model for each of the respective automotive modules, a step of associating, to the assembly model, at least two operations, wherein the respective operations require respective instantiations distinct from each other of the assembly model, a step of mapping the models of the automotive modules in finite elements and of mapping the links in finite elements to create at least one mapping in finite elements for each of the respective models and at least one mapping in finite elements for each of the links, wherein, in the mapping step, a first model mapping in finite elements and a second model mapping in finite elements distinct from the first model mapping are created for at least one of the models, wherein each of the instantiations of the assembly model includes a reference to at least one catalogue comprising respective test conditions for each of the at least two operations, wherein the respective test conditions for each of the at least two operations include mapping requirements for each of the respective models, wherein the mapping requirements for a first operation among the at least two operations include the first model mapping, and the mapping requirements for a second operation among the at least two operations include the second model mapping distinct from the first model mapping, a step of creating successively the instantiations of the assembly model, wherein the respective instantiations of the assembly model include the respective mappings of the assembly models in accordance with the mapping requirements of the catalogue, and a step of validating the instantiations of the assembly model by digital simulation, wherein the step of integrating the links is performed directly among the automotive modules, and not among the automotive models nor among the mappings of the automotive models, so that the assembly model is a geometric assembly of the automotive modules and the links, and does not include an assembly of the models, nor an assembly of the mappings, and successive instantiations of the assembly model do not require recalculating the mappings for the at least one model having first and second mappings, nor reintegrating the links, and wherein the step of creating successively the instantiations of the assembly model includes creating respective instantiations of the links by referring to a link instantiation catalogue that sets forth, for each link, a correspondence between said link and respective link parameters respectively corresponding to each of the at least two operations.