• The purpose and functions of the system should be described. If a system has more than one functional mode of operation (e.g., an aircraft, a search and tracking radar, etc.), the requirements in each mode should be identified separately. Requirements for alternative modes (i.e., redundancy) or standby modes of operation should also be identified.

• The main performance, safety and physical characteristics should be listed in order of importance. Acceptable limits of satisfactory performance should be stated so that failure criteria can be established, and any acceptable performance degradation that still allows a limited operational capability should also be defined. The latter is often important when performing a Failure Mode and Effects Analysis (FMEA), which is described in Chapter 6. Any physical constraints (e.g., size, weight, etc.) may be important, for example, when considering the scope for redundancy, or when assessing the severity of handling as a failure cause.

• Requirements for the specified reliability characteristics (reliability, Mean Time To Failure [MTTF], availability, failure rate, etc.) should be stated and quantified along with the time period, or other variables for which the requirement applies. If reliability requirements are specified individually for major sub-systems (rather than as an overall system requirement), the relevant data for each sub-system should be assembled accordingly.

• The specified conditions of use for the system (and sub-systems, if appropriate) should be stated, including operating states, environments, time intervals, maintenance policies, etc.. The sequence of use conditions during the period for which reliability is to be assessed is termed the operational duty cycle, and is described more fully in Operational Duty Cycle.