MDT ( Modified Data Tag ) is one bit of the attribute character, If it is off (0), it indicates that this field has not been modified by the terminal operator. If it is on (1), It indicates that field has been modified by the operator. Only when MDT is on, will the data of the field be sent by the terminal hardware to the host computer (i.e. to the applcation progam in end). An effective use of MDT drastically reduces the amount of data traffic in the communication line, thereby improving performance significantly. Therefore, BMS maps and CICS application program should be devloped based on careful considerations for MDT.