It is written in the manuals that CAT 3 a/b can be flown with 1) a decision height not lower than 100 ft, ....

Question

..... or no decision height, and 2) a runway visual range of not less than 50 meters (for CAT 3 b). So, why do they give a decision height requirement, if, in the next sentence, it is written that there is no decision height actually needed?

Answer 1

It depends on the qualification of the flight crew.  Some pilots are authorized to go down to 0/0 and some are only authorized to go down to 50' RA.

Answer 2

There is no DH only if further  training and equipment requirements are met.