Determining required properties with Schema.org vocabularies

[kdale]

Hi group,

I am updating from the data-vocabulary.org vocabularies to use the vocabularies from schema.org and as a rule, have followed the recommendation of schema.org vocabularies not being a 'all or nothing' deal, but including as much data as we have available for a particular type. 

My question: the google data-vocabulary help pages indicated which properties were required (for example http://www.google.com/support/webmasters/bin/answer.py?hl=en&answer=164506) but there is nothing (that I've found) at schema.org indicating which properties (if any) are required for a particular vocabulary. How do you know which properties are required for a given vocabulary, if any?

For example, I am using the http://schema.org/MusicAlbum vocabulary, and within for a track listing, the http://schema.org/MusicRecording vocabulary. I did not have duration or url included per track and when using the rich snippet validation tool (which I know is not 100% caught up with schema.org), it gave me warnings that these two properties were absent. Where in the docs are these flagged as required or, are the warnings to be taken as more of a suggestion i.e. 'your data would be more robust and meaningful with these properties...'

Much thanks!

Kris