When you validate or save a mapping, the Designer verifies that the definitions of the independent objects, such as sources or mapplets, match the instance in the mapping. If any object changes while you configure the mapping, the mapping might contain errors.
If any object changes while you are not configuring the mapping, the Designer and other PowerCenter Client applications track the effects of these changes on the mappings. The Repository Manager displays the status of mappings, so you can see if a mapping is valid or not. If you notice that a mapping is invalid, you can open the mapping and validate it to see the error messages in the Output window.