Using PACK ON while editing and saving members in a dataset helps us  utilize the space more efficiently. But Packed format is not supported  while processing these members, say for example compiling a cobol program in packed format may fail. To save a member in Packed format you can give command line command To unpack the member you can edit the member and issue command line command and save it to unpack it. You can know which format your member is saved in by issuing the command line which will list various other information including the format in which your member