When you develop a mapping, you must configure it so the Integration Service can read and process the entire mapping. The Designer marks a mapping invalid when it detects errors that will prevent the Integration Service from running sessions associated with the mapping.
The Designer marks a mapping valid for the following reasons:
Connection validation.
Required ports are connected and that all connections are valid.
Expression validation.
All expressions are valid.
Object validation.
The independent object definition matches the instance in the mapping.
Data flow validation.
The data must be able to flow from the sources to the targets without hanging at blocking transformations.