In iVI 4.0 and later it's possible to blend meta-data sources together so you can get some data from iTunes for example and the rest from TVDB. A common use of this would be to get show descriptions and images from iTunes and the actor and director information from TVDB.
The order of the meta-data sources (TVDB, iTunes, Movie DB) is important. You can drag meta-data sources up or down the list. The ones at the top of the list will be checked first, then the next source and so forth.
For movies and TV shows there are three options to choose from :
First Source : The default option. iVI just uses the top source on the list to find meta-data
Fill In : When this option is selected then iVI will take all the meta-data it can from the first source. Then it will check the next source and fill in any gaps from the first source and so forth down the list of sources
Overwrite : This is the opposite of Fill In. iVI will take all the meta-data the first source has to offer but will overwrite any fields it finds in the second source and so forth down the list.