I'm not sure at all about how could I ask for the exponent.

I'm afraid there is no satisfying solution, but to add this feature as a new AMCnumericChoices option.

Thank you for your answer, it would be most useful a feature to me.

Thinking for definiteness to the horizontal format, perhaps the layout could be implemented by adding <expdigits> (say) exponent rows with figures from 0 to 9 separated by some sort of horizontal rule from the preceding <digits> mantissa rows; maybe something along these lines, with digits = 3 and expdigits = 2:

+---+----- mantissa ------+
|   | 0 1 2 3 4 5 6 7 8 9 |
| + | 0 1 2 3 4 5 6 7 8 9 |
| - | 0 1 2 3 4 5 6 7 8 9 |
+---+----- exponent ------+
| + | 0 1 2 3 4 5 6 7 8 9 |
| - | 0 1 2 3 4 5 6 7 8 9 |
+---+---------------------+

Then the software logic for the mantissa could stay more or less the same as the current one, while the exponent could be catered for by multiplying the mantissa score by (codedExponent == correctExponent)?1:0.

Thank you, regards,
-- Davide G.M. Salvetti

I'm sorry, in my example of course I forgot to add a <decimal separator> to the layout between the first and second row of the mantissa, and, by the way, the TeX boxes reading "mantissa" and "exponent" should be user customizable.