I define functionality as the capabilities of a system that its functional requirements describe. In other words, it tells me what the system can do. For example, I list actions, responses, and business tasks that the system must perform. Therefore, functionality links requirements to real system behavior. Moreover, it guides design, testing, and delivery.
I view functionality from the user’s point of view. For instance, I use cases to show end-to-end behavior. Thus, I see the system as a black box. Consequently, I focus on interactions between actors and the solution. In addition, I use use cases to scope and structure a project. Furthermore, I refine use cases during ongoing product development. As a result, I maintain a clear picture of the user-level functionality.
I also distinguish functionality from features. A feature often bundles several functions. However, a function is a specific capability. Therefore, I split requirements into loosely coupled units of end-to-end functionality when possible. Then, teams can implement these units independently. For example, one team can build a checkout flow while another builds the product catalog.
Nevertheless, architectural and organizational constraints sometimes prevent neat splits. For example, shared platforms, layered architectures, or specialist teams create overlaps. Consequently, different teams must collaborate to implement intersecting functionality. In that case, I recommend that product owners coordinate requirements closely. Alternatively, I may form a dedicated team to manage the overlap. Meanwhile, I ensure each team keeps a shared understanding of the requirements.
I write functional requirements clearly and testably. Therefore, I include acceptance criteria for each functionality. Also, I link functionality to information models and dynamic views. For example, I connect data entities to the behaviors that use them. Moreover, I document interfaces and common business entities that affect multiple use cases. As a result, I reduce ambiguity and rework.
I choose my RE process based on how fixed the functionality is. If the customer requires a fixed contract, I follow a prescriptive approach. Consequently, scope comes before cost and deadlines. Conversely, if stakeholders only know goals at the start, I use an explorative approach. In that case, I explore concrete requirements iteratively. Therefore, functionality may change over time. Moreover, stakeholders must give continuous feedback.
I consider the target facet when I plan it. For customer-specific projects, I tailor it to one client. Conversely, for market-oriented products, I design functionality for many users. Therefore, I weigh delivery reliability, quality, cost, and scope differently. For example, in high-reliability contexts, I prioritize correct functionality over adding new features. Meanwhile, reporting and status tracking help me detect deviations early.
I keep communication simple. First, I define what the system must offer. Second, I detail how actors will interact with it. Third, I add acceptance criteria and test cases. Finally, I coordinate teams and product owners to ensure consistent implementation. In short, functionality represents the actionable capabilities of a system. Consequently, I treat it as the central bridge between requirements and delivered software.