Guys - 63 amp breakers are standard IEC ratings and have nothing to do with anything other than it is part of a preferred number series that is based on a geometric distribution of capacities from small to large.
Think of it this way.
In the US, you can buy a 1/4 HP motor, a 1/3HP motor, a 1/2HP motor, a 3/4HP motor, a 1HP, 2HP, 3HP, 5HP, 7.5HP 10HP, 15HP, 20HP, 25HP, 30HP, 40HP, 50HP, 60HP, 75HP, 100HP.
That is 19 different motor HP ratings between 1/4HP and 100HP.
If the DIN standard is used for defining power, they do a geometric distribution of power capacities between 1/4HP and 100HP resulting in a fewer number of motors to efficiently cover the same power range.
Using the R5 DIN 323 distribution you would have
.25HP
.40HP
.63HP
1HP
1.6HP
2.5HP
4HP
6.3HP
10HP
16HP
25HP
40HP
63HP
100HP
Same range of power capacities covered with 14 motors.
If you want finer resolution, you go to the R10 scale:
1
1.25
1.6
2.0
2.5
3.15
4
5
6.3
8
10
Each increment is an approximately equal percentage increase in power / capacity / etc.
R20 goes like this:
1
1.12
1.25
1.4
1.6
1.8
2
2.24
2.5
2.8
3.15
3.55
4.00
4.50
5.00
5.60
6.30
7.10
8.00
9.00
10.00
There is also an R40 series (finest)
I have heard that detents in european machine tool adjustments are set based on these same numbers.
