Sales and Operations Planning (S&OP) is a critical business alignment process for industries with large product portfolio. Indeed, high variety is a driver of complexity and results in a poor planning efficiency. Diagnosing the root causes of the process failure starts by assessing the maturity of its actual technical and organizational performance. The existing body of research lacks context-related maturity assessment frameworks that measure the maturity regarding activity-related dimensions. In this paper, we intend to propose a new S&OP maturity assessment model that considers the critical attributes linked to the context of mass customization production systems. Our review of existing contributions on maturity assessment combined with the results of workshops organized in a major automotive company ended-up on a suitable context-related design. The new framework has been tested within the automotive company by interviewing major decision makers and operational teams involved in the S&OP process.