For any filename, input or output, if the value of the variable is 'default' (case insensitive), the actual filename is constructed within the routine that will use the filename. The name is constructed by the same rule as that for the ${DEFAULT} environment variable (Section 2.1.4). The only difference between the two cases is that SET FILE = 'default' may not work as expected if the TOP variables defining the filename change between the SET command and the command that will use the filename, whereas SET FILE = '${DEFAULT}' will work as expected because the filename FILE is actually constructed during the SET command.4.2