Importing and Parsing a Flat File into webMethods using Flat

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I’m having problems in importing and parsing a fixed length (80 characters) flat file into webMethods. How can I filter only certain records with specific record identifiers? How can I create a default record for storing records which do not match the record identifiers set in the Flat File schema? How can I prevent unwanted data and information from appearing in the undefined data field? Help in resolving the above issues will be greatly appreciated.