As a prominent part of manufacturing system, assembly system provides a platform for increasing efficiency while delivering various market demands. However, due to the lack of a unified and clear definition of flexibility in assembly systems, the recognition of optimal flexibility in assembly system without clashing with efficiency still remains elusive. In order to establish a sound basis to discuss the characteristics of flexible assembly and to address the question of reaching optimal flexibility, this paper makes use of a case study performed in five manufacturing plants. The study proposes a clear definition for flexible assembly and identifies six enablers for flexibility in assembly systems. Further in this research the applicability of few different types of manufacturing flexibility in assembly system is investigated. The paper concludes with discussions and suggestions for future research.