Which of the following is NOT a category of availability?

To understand why the answer provided suggests that "System availability" is not a category of availability, we need to look closely at the definitions of the various terms related to availability in the context of systems engineering.

Operational availability refers to the capability of a system to be available for use when required, considering all factors that can affect availability, including maintenance and downtime. This category places a strong emphasis on the system being ready and functional during its operational phase.

Inherent availability focuses primarily on the system's capability under ideal conditions, often excluding any external factors such as maintenance or logistics. It provides a baseline idea of how well the system can perform its intended function without considering other variables that might impact its operation.

Measured availability considers actual conditions and is usually derived from empirical data over time, providing a quantitative measure of how available a system is when accounting for actual performance and history of failures or maintenance needs.

While "System availability" might have been used colloquially to refer to general availability, it does not represent a formal recognized category of availability within systems engineering frameworks, unlike the others listed, which each have clear definitions and criteria. This distinction sets it apart and validates the choice as not fitting into the structured categories typically recognized in the field.