Adding on to this, MID and MOD need not be of same size. MID corresponds to those parameters which the DC will take as Input to retrieve the date from IMS database. MOD will correspond to the data output to the IMS screen.
In my current assignment I found only 1 difference between MID and MOD.
MOD contains attribute parameters for the fields and error messages to be displayed at the end of the screen. Hence, the length of MOD is more than the MID in this case. The modules are used for Inquiry screen.