Any telecommunications or information system that is defined as a national security system (FISMA 2002) because it processes any information the loss, misuse, disclosure, or unauthorized access to or

Mission sensitive information refers to the data itself, not the system that processes it.

A mission critical system is specifically defined as a telecommunications or information system, often a national security system under FISMA, where the compromise (loss, misuse, disclosure, unauthorized access, or modification) of its processed information would have a debilitating impact on the agency's mission. The focus is on the system's importance to the mission.

A Major Application is a broader category under FISMA for applications requiring significant management attention, but it doesn't specifically convey the 'debilitating impact' aspect on the mission in the same way 'mission critical system' does.

An Access control System is a type of security system that enforces access policies, not a general classification for systems whose compromise would debilitate a mission.