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Within each content type class in the node.inc file, you are able to write function calls specific to each type. These are only necessary when data needs to be manipulated in some way between the source and destination databases. For example, if data types don’t match up between the source and destination, you can call prepareRow() to change the data type of a specific field. In the hr’s node.inc, prepareRow() is called a few times when a varchar needs to be converted to an integer; prepareRow() changes a “Yes” to a 1 and everything else to a 0. This function is called before the data is saved to the destination database. You may also write custom functions which are called before prepareRow(), and call them using ->callbacks(array($this, 'myFunction')) right underneath addFieldMapping in the class declaration. Note that callbacks are called after prepareRow(), but they also have less going on behind the scenes than prepareRow() does. For more information on callbacks and field mappings, see Drupal’s documentation here.