Alert height is aircraft specific and is related to its fail operational design.
a simple example will illustrate its significance.
Say we are conducting a CAT 3a approach, and our alert height is 100’. Our FMA is indicating that we are CAT 3 capable.
Scenario 1: At a height ABOVE our alert height we have a failure that has downgraded our approach capability to CAT 2. (Eg 1 autopilot failure). We are visually ‘Alerted’ about this via a change on the FMA showing CAT 2. we are no longer capable of CAT 3a so we go around or if we are sufficiently above the runway, and have the required RVR, we can revert to a CAT 2 approach and continue.
Scenario 2: At a height BELOW our alert height the same failure happens. The aircraft will not ‘alert’ us of the failure. we are left ignorant of the fact that the system redundancy has downgraded. The aircraft can still conduct an automatic landing and considering the few seconds left in the approach, another failure that will render the system unable to autoland is highly improbable. Just incase a failure of that nature does occur though, we get an Autoland warning light. No real time to analyse the failure so the system only flashes one important red light in your face should the improbable occur.. If you’re still in the soup, Go around.