In the event of a fault the incomer (63A) will trip before the 100A MCCB. If the incomer is feeding other circuits they will trip rather than your 100A MCCB. Your circuit will be limited to 63A.
A 100 or even 200A bus is usual in a distribution board. Feed it from 32A and thats the maximum you can get from it. Above 32A your MCB will trip. (eventually and all of that stuff in accordance with I²t curves)
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