Zn is addition mod n on the set of integers from 0 to n-1.

Un is multiplication mod n on the set of integers
from 0 to n-1 that are relatively prime to n.

If you select Zn, the box on the right displays with Z selected. Enter the
number of elements. Your computer may
not be able to handle over 61 elements.

If you select Un, the same box displays with U selected.