Often there is a set of standard transformations between two document types that are captured in a given map. If a small customization is required, it comes in handy to have one map reference another map. The parent map would be the standard map, and the child map is the map where the small customization must occur. This customization is done only in the child map, and any future changes to the parent map are automatically reflected in the child map. This avoids the maintenance problems of making complete copies of maps and having them subsequently change.
Another useful case is where you have small maps that transform a portion of a document, and you want to use these to build up a larger map. These small maps (for example corresponding to EDI segments) can be used to compose several different larger maps.
When a map inherits from another map, this forms a parent/child relationship between the maps.
A parent map and child map can both be associated with the same structure. In this case, both maps have exactly the same map elements. If there is no value expression associated with a map element in the child map, the value expression (if any) associated with the parent map is used. However, if there is a value expression in the child map, the Output Value expression in the parent map is not used.
It is also possible that the parent map is associated only with a portion of the structure of the child map. In this case, when associating a parent map, you specify the elements in the child map that anchor the parent map.