I know that currently protectorates are 'not optimal' so I havent entered into any agreements of this kind, but I am wondering what they are supposed to do?

Do the provinces of the protectorate count towards the required 50 for example?
Are they required to give you tribute as long as they are a protectorate?
Military aid (ie an alliance) ?
Follow your 'external policy', ie declare war on those you declare war on etc?
Something else?