This menu allows you to keep the data from the existing database. To preserve the data in a field, you must map it to a new field, where the data will appear. Any field can also be left blank, with no information copied into it. Any old field not mapped to a new one will be lost and all its data removed.
You can only map fields of the same type, so each drop-down menu will have different fields in it. Also, you must be careful not to try and map one old field to more than one new field.