Which category of system requirements does supporting 25 users online simultaneously fall under?

Supporting 25 users online simultaneously falls under the category of performance. Performance requirements define how well a system functions under specific conditions, particularly regarding speed, responsiveness, and throughput. In this case, the ability for the system to support 25 users concurrently reflects its efficiency and overall responsiveness when accessed by multiple users at the same time.

This aspect directly relates to how the system manages resources and handles multiple transactions or requests, which is critical for ensuring that all active users have a satisfactory experience without delays or interruptions. Since performance considerations are vital in multi-user environments, this aspect helps to determine if the system can meet the expected load without degradation in service quality.

While scalability refers to the system's capacity to grow and accommodate increased load in the future, supporting a certain number of concurrent users is fundamentally linked to how the system performs at that current load. Usability pertains to how easy and intuitive the system is for users, and functionality relates to the specific features and behaviors it provides. However, the focus in this scenario is specifically on the system’s ability to maintain efficient operation with multiple simultaneous users, hence categorizing it under performance.