Values can be null   Select this option if values in the column can be NULL. If the column is based on a domain, you can retain the domain's nullability or override it for the column. See The NULL default.

Values cannot be null   Select this option to allow duplicate values, but not allow NULL values in this column.

Values cannot be null and must be unique   Select this option if values in the column cannot be NULL and must be unique. This option is disabled if you specified a spatial type for the column's built-in data type or if you specified a domain whose base type is a spatial type. See Column Properties window: Data Type tab.