Assign an INFOOBJECT - direct transfer, no transformation
Assign a constant eg. If you are loading data from a specified country from a flat file, you can make the country (17s) as a constant and assign the value explicitly
ABAP routine eg. If you want to do some complex string manipulation, assume that you are GETTING a flat file from legacy data and the cost CENTER is in a field and you have to "massage" the data to get it in. In this case the use of an ABAP routine is most appropriate
Formula - for SIMPLE calculations usc formula eg. If you want to convert all lower casc cl~uractcrst o upper case, use the TOUPPER formula. You can use formilla builder to help put your formulas together.