What is the relation between modal base and ordering source?

I edited the question again. Here is an explanation for epistemic modal,

John must have the flu.

a. Epistemic Modal Base (MBepis) = { John has a fever, John has a cough, John did not get a flu shot, ...}

b. Doxastic/Streotypical Ordering Source (OSdx/st) = {Flu leads to a fever, Many people in town are suffering from flu right now, People suffering from the same symptoms in the same town all have the same disease, ... } (Portner 2009:pp.64-65)

I just can't figure out what is the difference between modal base and ordering source? It seems like they're both a set of propositions right? Do they have any relation? And what does it mean "modal base and ordering source are both function from worlds to sets of propositions"?

• You really need to explain a lot more in your question. For example, readers need to know that you’re asking about Angelica Kratzer’s theory of modals. – Araucaria - him Dec 2 '20 at 12:06
• I don't know the theory you're asking about, but it strikes me that set a contains observations about John and set b contains general knowledge / assumptions that lead you from those observations to the conclusion that John has the flu. Since your conclusion is then an evidence-based inference it's appropriate to use "must". There looks to be something missing from your last sentence but it may just mean that the elements in both sets are abstractions. – rchivers Dec 2 '20 at 13:55
• @rchivers Thank you for your kind explanation which makes it much clearer! Thank you so much! – ronghe Dec 5 '20 at 14:50