This value records what the power unit's status was immediately prior to its current status, letting an administrator see the previous state alongside the current one to understand a recent transition, such as a unit that just failed. Administrators use this together with the current status field and the associated notification to confirm exactly what changed, for example distinguishing a newly-failed unit from one that's been failed for a while. For example, seeing this previous-status value as normal(2) while the current status shows fail(3) confirms this power unit just transitioned from working to failed, rather than having always been in a fault state.