Some questions about sporadic server in seL4

I have read Sprunt’s paper in 1989 and Stanovich’s paper in 2010 about sporadic server.
I have a few questions when looking at the source code:
1.What are the requirements for setting a round robin type of thread, and where are round robin threads used for?
2.whats the difference between sporadic and constant band-width? I didnt see related discription in those paper,is there any explanations?
3.In the refill_budget_check function code, when the head refill is not sufficient, how to ensure the following refills is activated by current time ?