How do I find the Video Downloads associated with a DVD?
You can use the RelationshipType parameter with the RelatedItems response group
to find Video Downloads associated with a DVD (or the DVD associated with a Video Download or the Blu-Ray associated with a DVD, etc.).
For example, consider the DVD of the film Serenity. The ASIN of the DVD is B000BW7QWW. To find the Video Download of the film, do an
ItemLookup on the DVD's ASIN (B000BW7QWW) with RelationshipType=AuthorityTitle:
>>> View ItemLookup for B000BW7QWW
In the RelatedItems element, you find the ASIN of the parent - B000AGY7DA.
Do basically the same ItemLookup on the parent ASIN (B000AGY7DA):
>>> View ItemLookup for B000AGY7DA
In the RelatedItems element you'll see the ASINs of the DVD, Blu-Ray and Video Download versions of the album.