You can define correspondence between the source and destination (target) fields also known as mapping. The program automates this procedure using fields with the same names. Also, you can change the default mapping by using the "Map" and "Unmap" buttons.

To unmap some correspondence select it and click "Unmap". To map select one field in the source table and one field in the target table then click "Map".

Also, the mapping dialog box allows you to describe additional conversions for the source data during the migration or import process.
You can specify a function for column conversion. To do it, enter the name of the function or an expression in the 'Conversion expression' field. If you use the name of a function, you will get an expression of the following type: FUN(column). You should use the %s marker to specify the position of the parameter for an expression, e.g. SUBSTR(%s,1,5).
You can specify the explicit conversion of types to the Integer or Float value. It makes sense when you need to remove non-significant zeros, etc.

See also: how to use database built-in functions with mapping feature.