For this example federation of three models, the SoS Library is a direct usage of the SoS requirements model and an indirect usage of the SoS architecture model. Is this the preferred method of using the library or is it preferable to have the SoS Library as a direct usage of the SoS architecture model? Are there pros and cons of either approach? In this case, the SoS Library contains customizations and stereotypes that are needed by the SoS archtiecture model.