Cross validation prevents the users from entering the invalid flexfield combinations. This is applicable to all the responsibilities.
Yes we may.if we register some other columns then we may use them as flexfields.But this is not recommended by Oracle.
Key Flexfield is used to describe unique identifiers that will have a better meaning than using number IDs. e.g a part number, a cost centre etc.
Desc Flex is used to just capture extra information.
Key Flexfields have qualifiers whereas Desc Flexfields do not.
Desc Flexfields can have context sensitive segments while Key flexfields cannot.
Context Sensitive segments in descriptive FFs are segments which can be made available or not based on the values entered by the Users in a 'Context' field. This helps in building a value based logic for the FF.