When you use the number of key fields and a value to specify the FROMKEY or TOKEY parameter, the copy command ignores the null values. The command uses only the buffer default values for values that are actually null for the comparison.
When you specify *BLDKEY on the FROMKEY or TOKEY parameter, none of the *BLDKEY values can refer to a null-capable field. If they do, the system sends an error message.