3.2.1 Category-IIIA Operations. (as per ICAO)
A precision instrument approach and landing with Decision
Height (DH) lower than 100 ft or no DH  and RVR not less than
200 M.

3.2.2. Category-IIIB Operations. (as per ICAO)
A precision instrument approach and landing with Decision
Height (DH) lower than 50 ft or no DH and RVR not less than
50 M.

3.2.3. Category-IIIC Operations. (as per ICAO)
A precision instrument approach and landing with no Decision
Height (DH) and no RVR.
Note (1): Where the DH and RVR do not fall within the same
Category, either the DH or the RVR may determine in which
Category the operation is to be considered. The operation will be
in the Category with lower minima. 
Note (2): Autoland is not CAT III.  An automatic landing
system is only equipment providing automatic control of 
the aircraft during the approach and landing and is not related

to particular weather conditions.  This system is mandatory for all CAT III operations. However, it is a common practice to perform automatic landing in good visibility but in that case, the ILS performance must be sufficient and ILS signals protected.

3.3. Landing Categories:
Each FMGC computes its own landing category: CAT1, CAT2,
CAT3 single and CAT3 dual and displays the corresponding
landing category on the FMAs.
Each category depends upon the availability of aircraft systems and
functions. When the landing category downgrades, a triple clic
aural warning is activated.

3.3.1 Fail-Operational Automatic Landing System
An automatic landing system is fail-operational if, in the event of a failure below alert height, the remaining part of the automatic system allows the aircraft to complete the approach, flare and landing. A CAT 3 DUAL system is a fail-operational automatic landing system. Note: In the event of a failure, the automatic landing system operates as a fail-passive system.

3.3.2 Fail-Passive Automatic Landing System
An automatic landing system is fail-passive if, in the event of a failure, there is no significant out-of-trim condition or deviation of flight path or attitude but the landing is not completed automatically. A CAT3 single system is a fail-passive automatic landing system. Note: With a fail-passive automatic landing system the pilot assumes control of the aircraft after a failure. Below 100ft (RA), the FMGS freezes the landing capability until LAND mode is disengaged or both APs are off.

Therefore a failure occurring below 100ft does not change the category of the system
3.4. Decision Altitude (DA) / Decision Height (DH).
A specified altitude/height in the precision approach at which a missed approach must be initiated if the required visual reference to continue the approach has not been established. i) Decision Altitude (DA) is referenced to mean sea level
　
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