By default, the same type map can be applied in two directions: forward and backward. The backward direction can be set by selecting the Run type mapping in reverse order check box in the third step of the Model Transformation Wizard. This is useful, when there are two related opposite transformations for some domain; for example, the same type map is used for both UML to XML schema and XML schema to UML transformations.
If you want to limit the directions, in which type map can be used, you can set the defaultDirection tag for your type map package. Possible values are forward, reverse, and both (default).
The direction can also be limited on a per-rule basis. This is controlled by setting the direction tag on the type map dependency. Then the mapping rule is excluded from the rule set when the type map is run in a different direction than specified for this rule.