The Sunstar Elite are the personal guards of the Emperor. They are reserved strictly for his protection, and the protection of those who he chooses to grant their power. There may be some extremely high ranking officials within the Empire who are seen with Sunstar Elite guards and this is always done on the Emperor's own orders.
Originally the Sunstar Elite were only elite Blood Knights, thus paladins who provided the protection for the Emperor but since then, the ranks of the Sunstar Elite have diversified to include Death Knights, Warriors and Templars.
The Sunstar Elite mainly guard the Emperor wherever he is living and also where he went and where he is to go. Unlike the Blood Sun Elite which function mainly as palace guard, the Sunstar Elite go where the Emperor goes.