The metadata that you associate with a target can be an arbitrary format. There is no required structure. The only restrition is that it can't be more than 150KB or 200KB if encoded as based64. 

For example the metadata associated with the Cloud Recognition in Vuforia target in the sample is simply the text ..

samplebook1.json

Which is used by the ContentManager to call for JSON serialized metadata from server..

{"title":"Cloud Recognition in Vuforia","author":"Karina Borland","average rating":"4","# of ratings":"41","list price":"43.99","your price":"43.15","targetid":"a47d2ea6b762459bb0aed1ae9dbbe405","thumburl":"https://developer.vuforia.com/samples/cloudreco/thumbs/01_thumbnail.png","bookurl":"https://developer.vuforia.com/samples/cloudreco/book1.php"}

You could also use JSON for the target metadata if you'd like to use a format that can convey objects.