The mappings page controls where and how the values of a source attribute should be placed in the target attribute. By default this is a one-to-one mapping, but you can change it. For example, whatever the value is for company name for all objects in the source that have that value will be used to the fill the company name value for all objects that have that attribute in the target. However, you can edit and customize the mappings to fit any special needs.
As an example, suppose that you are doing an inter-forest migration where the target company has acquired the source company. The source company uses the Employee Number field as a unique identifier. However, the target company uses the Employee ID for this same purpose. We can modify the mappings to make this adjustment. First, we will make sure that the Employee ID field in the source does NOT get mapped to the EmployeeID field in the target as we plan to have a different source field mapping to that. We can just empty it out, or we could remove the entire mapping.
Now we will map the Employee Number source field to the employee ID target field. If you need to get more granular, you can specify the source and target object types that this should only occur with. Such as just user target objects. In my example, there is no need because this attribute is only found on user objects anyway.
You can control the number of items included in the window here. You can search for a particular mapping with the search box. You can filter the attributes by object type to narrow your listing.
Click on the advanced button for more options, including the ability to reset everything.