Tune your map for specific scenarios using <mapsource>

You can modify some default behaviors of the Mapper by modifying attributes of the mapsource element directly in a map source (.btm) file. There are currently three behaviors that you can modify:

· Optimize Value Mapping functoid code generation. You can modify the behavior that controls when a variable is used with if statements.

· Accommodate schemas with large footprints. You can change the way internal compiler nodes are used in large maps.

· Manage for-each usage with Looping, Conditional, and Value Mapping functoids. You can control where the xsl:for-each statement is used within the destination schema.

Source: Microsoft