What defines the number of digits in the FIS code numbers? Alice and MS have numbers with 7 digits, and there are others with only 5 digits.
I don't know. It looks like the leading digits, all but the last three, are
almost a country code.
The Swiss all start 516, the Austrians 56, the Germans 205 or 206, the Italians 297 or 299, the Canadians 105 or 107, France 197.
But the USA numbers are all over the place: 6535, 538, 539.
Maybe that is just a coincidence and it just reflects where or when their number was issued. In the old days before centralized computer systems, many organizations had a different pool of ID numbers that each office could give out, so they did not have to deconflict them. (Notably, the US Social Security numbers were handled that way -- no idea if they still are.)