The difference is this:
The source informat is used when reading the value from the Excel or text file. Use the proper INFORMAT to interpret the raw value from the data source into a column in your SAS data set.
The output informat is the INFORMAT that you want to associate with the column in your output data set. That INFORMAT would be the default INFORMAT used for future input operations (such as VIEWTABLE, as pointed out). Note that in the data grid within EG 4.2, the INFORMAT behavior is *not* used.
See http://susanslaughter.wordpress.com/2010/02/09/eg-4-2-tip-farewell-to-informats.
Chris
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