This window controls profile merge operations.
The source profile is the profile that information is being copied from. The destination profile is the profile that information is being added to (or replaced).
For many categories, there is an option to select Replace or Merge. When Replace is chosen, the contents of that section of the source profile completely replace that section of the destination profile.
When Merge is chosen, items in the source profile that do not yet exist in the destination profile will be added to the destination. If an item exists in both profiles, the value from the source profile will replace the one in the destination profile. Items that exist only in the destination profile will be left as they are.