Very often we use DDIC reference to create entity and entity sets. For some standard document numbers, we always have leading zeros (alpha exit). And its not ideal to show that to user on his screen. By default, SAP does not support these conversions. Or as Anubhav says:
'There is nothing like magic' :)
You have to tell your model provider to do that for you. One of the quickest and easiest method is to redefine your 'define' method in MPC extension class (*MPC_EXT). Get the property (field) of your entity set and call the method set_conversion_exit.