Cabin Alt
Air is a mix of gasses, oxygen is one. As altitude increases the total pressure reduces and the component pressure of each gas reduces accordingly. At about ten thouand feet the lower pressure has significant effect on some bodily functions and supplementary oxygen is required. Thre is a discernable reduction in some bodily functions at lower levels night vision and colour perception. In a well designed pressure regulator supplying a mask the fraction of oxygen increases with altitude until about 25.000' pure oxygen is available. Above that level even pure oxygen fails to reach the partial pressure exerted by the oxgen at lower levels and pressure breathing must be used to supply pure oxygen at greater than surface pressure.
In the Vs a lower cabin pressure level called "combat" could be selected to reduce the impact of cabin pressure loss. The switch was next to the co pilots right elbow and accidental activation was a frequent source of annoyance