Functional requirements are not enough, and a Project Manager must not only have requirements but understand what is intended by them. A good Project Manager will do this in two ways, requirements decomposition and Concept of Operations (CONOPS) which are agreed upon by the stakeholders. Whether you are in an Agile or Waterfall development, the upfront work will pay off. Not to say that things cannot change, but all changes should be documented as to the change, the effects, and why via Configuration Management, another blog.
Many times, you will be given a set of requirements and expected to deliver on them. These requirements are often non-specific and difficult to test. Remember that the requirements are not only what problem you are solving, but you will also need to know how to prove and sell off your final product. This is where CONOPS is essential, as well as some more details of the requirements that will serve you well. It is always worth the time to sit down with the stakeholders, especially the users, and the manufacturers to make sure everyone knows what the final product will be. This is also the time to consider testing, using the requirements and the CONOPS. For every requirement, ask everyone what they think that means, then write out and document the answers. I can cite several examples of users and searches, where the requirement said that the system shall be able to execute ten searches for 100 users within 10 seconds at once. If you test this by performing the same ten searches using 100 simulated users, you will sell off this requirement. Still, in real usage, the system will crash when ten people in actual usage execute three different searches. Had you sat down with the users and developers, they would both understand that the system, in fact, needs to support multiple people doing random searches, which will drive your test criteria. The stakeholders should also be present when you lay out your test plan so that you have an agreement on what sells off the system and what would not.
In the end, the up-front time spent in truly understanding the requirements, from the stakeholder’s view, will lead to better results and cost savings, as you get to make the system once. There have been many contracts, when they just ask to sell off the system, and will not make CONOPS a deliverable for which they do not want to pay. Still, as a Project Manager, you will want the CONOPS as well as a sit down with stakeholders to understand what they meant by the requirements and how they expect to use the system. Your other option is a significant cost and schedule overruns and angry customers.