1. A computer program product, tangibly embodied in a computer-readable storage device comprising instructions, which when executed on a processor, cause the processor to execute a method for storing customization data in a runtime repository, the method comprising: storing a repository object in the runtime repository, the repository object comprising one or more model entities; storing one or more variants for the repository object in the runtime repository, each of the variants representing a customization of the repository object according to a specific criterion, and each of the variants for the repository object being arranged in a first hierarchy; storing one or more layers for the repository object in the runtime repository, each of the layers representing a customization of the repository object according to a customizing party, and each of the layers being arranged in a second hierarchy, wherein the first hierarchy for the variants and the second hierarchy for the layers provide a multi-dimensional representation of customizations of the repository object, such that each of the customizations of the repository object is identified by one of the variants of the first hierarchy and one of the layers of the second hierarchy; storing a first set of customization values for one or more of the model entities in the runtime repository, the first set of customization values being stored in a first variant within the first hierarchy and in a first layer within the second hierarchy; and storing a second set of customization values for one or more of the model entities in the runtime repository, the second set of customization values being stored in the first variant within the first hierarchy and in a second layer within the second hierarchy.