Metrics, collections, agent workload, and XML files

I'm trying to understand the relationship between the agent uploads, metrics, and blackouts.  

Does OEM stop collecting all data from the EM agents during a blackout?  

Does the EM agent always collect the same set data from all databases?

Are the size of XML uploads affected by metrics definitions?

Are metric definitions merely a filtering process of that standard data that is managed purely between the OMS and the repository?  

The collection intervals under this model reflect aggregation and reporting intervals, but the same data is colllected by every agent for every database all the time.