Hi Allen,
Its look logically correct to me because if you are defining given domain(with default value 'X') in new data element and further, this data element defined in primary key field of the table then whenever any record will be inserted to the table without passing any value to given field then it will be set with default value equal to 'X' which is not correct as per SAP.
If you have nay query,please let me know the same.
Regards,
Sudeesh Soni