Software architecture mainly focuses on the high-level structures of the proposed software, and how to document these structures. A documentation method that represents an incomplete picture is one reason for inadequate requirements. This leads to requirements engineers wasting their time arguing over what to do and how to do it. Four documentation methods are frequently used in order to document stakeholders' statements, particularly for representing the functional perspective, namely, Natural Language (NL), Data Flow Diagram (DFD), Use Case Diagram (UCD), and Activity Diagram (AD). This research was carried out using the electronic market application domain as a test context. A controlled experiment was conducted among 158 participants, comparing among NL, DFD, UCD, and AD methods, which aimed to find out which requirements documentation method is more effective, helpful, and easier to comprehend. The results from this empirical study reveal that the AD method is more effective, understandable, and easier to document the software requirements in the functional perspective. Furthermore, AD had better performance in representing the requirements engineering context, system context, and development context than the other functional documentation methods. These empirical results would help software companies and associated experts enhance the quality of their software products, as well as increase the chance of success of software projects.