Per the OTU, there's a truly amazing variation in type of government of the Imperium's planets, but there's also "Imperial Nobles" listed assigned to member worlds. What function do they serve? Liaisons? Governors? Members of whatever government the world has? Something else? 

How does the Emperor maintain any level of stability of income and levies with such an internal variety? Is there some standard policy of how to govern, or are High Nobility pointed at a volume of space and told to "Serve the Emperor's Interest"?