The requirements for the whole application are classified as follows according to the BABOK ver.1.6:
“Business Requirements are higher-level statements of the goals, objectives, or needs of the enterprise. They describe such things as the reasons why a project is initiated, the things that the project will achieve, and the metrics which will be used to measure its success.
User Requirements are statements of the needs of particular stakeholder or class of stakeholders. They describe the needs that a given stakeholder has and how that stakeholder will interact with the solution. User Requirements serve as a bridge between Business Requirements and the various classes of solution requirements.
Functional Requirements describe the behavior and information that the solution will manage. They describe the capabilities the system will be able to perform in terms of behavior or operations – a specific system action or response.
Quality of Service Requirements capture conditions that do not directly relate to the behavior or functionality of the solution, but rather describe environmental conditions under which the solution must remain effective or qualities that the system must have. They are also known as non-functional or supplementary requirements.
Assumptions and Constraints identify aspects of the problem domain that are not functional requirements of a solution, and will limit or impact the design of the solution.
Implementation Requirements describe capabilities that the solution must have in order to facilitate transition form the current state of the enterprise to the desired future state, but that will not be needed once the transition is complete.”
The user interface requirements are related to the user experience with the system. The requirements in this category look at the software usability, user friendliness, ease of use, colors to be used so that various color blind users could use the product with the same results and satisfaction as the other users, etc. In addition there are different user interface requirements if the application is Windows or Web-Based.
The Use Cases are not in the category of requirements. The use cases are a technique for modeling the system usage. They could be used when documenting functional and user requirements.
The user interface/experience requirements are better captured with screen mock-ups, prototypes, detailed descriptions, etc.
Hope this helps,