Two tools which are used for keeping track of software requirements:

• The system's border, its surrounding environment, and all the interacting entities are all defined in a system context diagram. Customers, external or internal systems, the organization's end users, and any vendors or suppliers providing third-party services are all identified in the system, which is plotted in the middle of the diagram.
• A functional decomposition diagram shows the primary functionalities of a business process or a system from the top down. You can use the functional decomposition diagram to break down what the system should perform when you  thinking about it. This view also aids in validating all of the system's functions. It's similar to an organisational chart, and your end user should have no trouble understanding it.

Read More: What is verification and validation in Software Engineering?