There is a particular height used by the manufacturer below which the systems are locked in a fail-operational mode, i.e., busses separated and so forth so that a failure will not result in a downgrade to fail-passive being annunciated. Boeing has traditionally used 200 feet for this purpose.
Then there is the alert height authorized by the authorities, and this is probably where a lot of confusion will settle in. The FAA did approve 50 feet as an AH for a long time; however, my current carrier is only approved to use 100 feet, and when I set up the CAT II/III program at my previous carrier, we were told the FAA was no longer approving a 50 foot AH for a new program (those already in existence being grandfathered, as always).
I can't elaborate on the approved AH under authorities other than the FAA.