Checkboard, the unaided manual Cat 3 landings were part of a research programme investigating pilots' capabilities after fail passive AP failure, to allow use of lower RVRs. This also considered FD HUD.
The trials looked at separate aspects of the RVR required to make a decision - above DH, and possibly a lower value in continuing with a manual landing below DH; the excluded option was for manual GA below DH.
The first use of adjusted regulation was UK CAA clearance for the Avro RJ with 50ft DH.
Generally Cat 3 operations were more consistent; enabling lower RVR because of less variability in the fog, opposed to Cat 2 where conditions are more associated with fog formation and dispersal or other reductions in visibility, snow, dust. In these, Cat 2, it is possible to decide to land and then find that the visibility was not valid for manual continuation as the conditions changed. Greater exposure time from higher DH and more lateral deviation to be considered.
Operational minima have a significant safety margin for the authorised approach, although Cat 2 has the lesser margin, and in extreme where it is possible to decide to land and then be wrong !