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Collections are a way for you to organize kata so that you can create your own training routines. Every collection you create is public and automatically sharable with other warriors. After you have added a few kata to a collection you and others can train on the kata contained within the collection.
Get started now by creating a new collection.
I was hoping to find this explanation after trying to fit the 'y' sample test you mentioned with the rest.
Re-posted as an issue.
BTW there's sample test with 'y' which should give a hint, but I agree that 'y' being/not being a vowel can come as surprise for people using different languages.